The Journal of Bone & Joint Surgery

Current Concepts Review | January 01, 2009

A Comprehensive Review of Partial Anterior Cruciate Ligament Tears

Michael J. DeFranco, MD¹; Bernard R. BachJr., MD²

¹ Harvard Shoulder Service, Massachusetts General Hospital, Yawkey Center for Outpatient Care, 3G, 55 Fruit Street, Boston, MA 02114
² Midwest Orthopaedics at Rush, Division of Sports Medicine, Rush University Medical Center, 1725 West Harrison, Suite 1063, Chicago, IL 60612. E-mail address for B.R. Bach Jr.: brbachmd@comcast.net

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Disclosure: The authors did not receive any outside funding or grants in support of their research for or preparation of this work. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.

The Journal of Bone and Joint Surgery, Inc.

J Bone Joint Surg Am, 2009 Jan 01;91(1):198-208. doi: 10.2106/JBJS.H.00819

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Abstract

A partial tear of the anterior cruciate ligament is characterized by an asymmetric Lachman-test result, a negative pivot-shift test, a low-grade KT-1000 arthrometer measurement (=3 mm), and arthroscopic evidence of anterior cruciate ligament injury.

The pivot shift test is the most important test in determining anterior cruciate ligament insufficiency. A positive test, independent of the grade, is indicative of a functionally deficient anterior cruciate ligament.

Nonoperative management results in an acceptable clinical outcome in the majority of cases.

Progression to knee instability (anterior translation and rotation) depends on the extent of the anterior cruciate ligament injury and the activity level of the patient.

Thermal treatment is not recommended for partial tears of the anterior cruciate ligament.

Symptomatic instability is treated reliably with anterior cruciate ligament reconstruction.

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Introduction

Isolated injuries of the anterior cruciate ligament account for nearly half of all knee ligament injuries and are among the most common causes of knee instability^1,2. In general, isolated injuries of the anterior cruciate ligament are defined as complete or partial tears. In many cases, however, partial tears are not diagnosed clinically, and they may mimic a variety of internal derangements of the knee³. Although not as common or as widely studied as complete ruptures of the anterior cruciate ligament^3-13, a partial tear of that ligament is a substantial injury requiring the same thorough clinical assessment in order to develop an appropriate treatment plan. Both the diagnosis and the natural history of partial tears of the anterior cruciate ligament are controversial issues. Consequently, this injury continues to be a diagnostic challenge and treatment dilemma for orthopaedic surgeons. The purpose of this review is to clarify the definition, clinical assessment, and strategies for treatment of partial tears of the anterior cruciate ligament.

Anatomic and Biomechanical Aspects

The anterior cruciate ligament has two functionally separate and distinct bundles: one is anteromedial, and the other is posterolateral^14-17. The femoral attachment of the anteromedial bundle is the center of rotation of the anterior cruciate ligament, which explains the isometric behavior of this bundle. The isometry allows the longer, thinner anteromedial bundle primarily to limit anterior translation of the tibia on the femur with the knee in flexion. It also contributes to stability in both internal and external rotation^17,18. The bulkier posterolateral bundle is not isometric. It limits anterior translation, hyperextension, and rotation^2,18,19. The oblique position of the posterolateral bundle provides more rotational control than is provided by the anteromedial bundle, which is in a more axial position²⁰. This continuum of tissue rotates in part because of the cam-shaped lateral femoral condyle.

The anteromedial bundle tightens in flexion while the posterolateral bundle relaxes. The posterolateral bundle tightens in extension and internal rotation. This reciprocal tensioning pattern suggests that partial ruptures can involve individual bundles, depending on the knee flexion angle²¹. The anteromedial bundle is more susceptible to injuries with the knee in flexion. Rupture of the anteromedial bundle may result in an increase in anterior translation in flexion, little or no increase in hyperextension, and little or no clinically evident increase in rotational instability¹⁹.

Hyperextension and internal rotation place the posterolateral bundle at greater risk for injury¹⁶. Rupture of the posterolateral bundle may result in an increase in hyperextension, an increase in anterior translation with the knee in extension, an increase in external and internal rotation with the knee in extension, and an increase in external rotation with the knee in 45° of flexion¹⁹.

Substantial interstitial damage of the anterior cruciate ligament occurs during partial tears. This damage is not always readily apparent on visual inspection of the ligament. Prior to ultimate failure, the ligament may elongate by >50% compared with its resting length²². As a result, it can be difficult to assess the true extent of injury and the strength of a visibly intact ligament. If one bundle deforms enough to rupture, then the other bundle will in all likelihood undergo permanent interstitial lengthening^22-24. The discrepancy between the macroscopic appearance and microscopic damage explains, in part, the difficulty in predicting which knees will become unstable and require reconstruction.

The anterior cruciate ligament does not have the capacity to heal sufficiently to regain adequate function after tearing^15,25,26. Healing is intimately related to blood supply, which primarily comes from epiligamentous tissue²⁷. The vascularity of the anterior cruciate ligament may be compromised at the time of injury or during subsequent episodes of instability²⁸; thus, progression from a partial to a complete tear is possible as a result of ischemic necrosis of intact fibers secondary to this disruption of the blood supply. Disrupted fibers are reabsorbed, remain in the intercondylar notch as nonfunctional fibers, or become attached by scar tissue to the surrounding synovial tissues⁹. This inadequate healing combined with substantial microscopic injury defines the functional incompetence of the partially torn anterior cruciate ligament. Clinically, an anterior cruciate ligament-deficient knee is manifested by recurrent instability, meniscal and chondral abnormalities, early degenerative changes, and impaired function^6,22,29-31.

Definition and Diagnosis

There is controversy regarding the definition of a "partial tear." The descriptions vary from bleeding at the femoral attachment of the anterior cruciate ligament to total rupture of the anteromedial or posterolateral bundle^3,32-34. According to the American Medical Association Injury Classification System (Table I), a partial tear corresponds to a grade-II (second-degree) ligament injury³⁵. Another way to describe the injury pattern is by referring to the specific anterior cruciate ligament bundle (anteromedial or posterolateral) that is torn. A partial tear of the anterior cruciate ligament can also be defined by estimating the percentage of the ligament that is torn on the basis of the findings on physical examination, magnetic resonance imaging, arthroscopy, or a combination of these modalities. However, each of these diagnostic measures has limitations, which will be discussed in the following sections. The percentage of torn fibers considered to define a partial tear of the anterior cruciate ligament has ranged from <25% to 75%^9,36-39. Furthermore, partial tears of the anterior cruciate ligament are sometimes referred to as high-grade (involving >50% of the ligament) or low-grade (involving <50%)⁴⁰. The most common site of partial tearing is near the femoral insertion, but tears can also occur within the midsubstance of the ligament or at the tibial attachment. Overall, we believe that a partial tear of the anterior cruciate ligament is defined by a combination of factors, including (1) an asymmetric Lachman-test result (as compared with the result in the uninjured knee)⁴¹, (2) a negative pivot-shift test^41,42 with the patient under anesthesia, (3) a low-grade KT-1000 arthrometer measurement (=3 mm)⁴³, and (4) arthroscopic evidence of an anterior cruciate ligament injury. An asymmetric positive pivot shift, regardless of the grade, is consistent with functional instability. When these criteria are used, the injury is being defined on the basis of functional instability, not the residual structural integrity of the ligament. If the four criteria are satisfied, then the patient is diagnosed with a partial injury of the anterior cruciate ligament and the anterior cruciate ligament is considered to be functional. If the interstitial damage to the fibers is severe enough to result in a positive pivot-shift test, then the anterior cruciate ligament is nonfunctional. The pivot shift test allows one to distinguish between an injured anterior cruciate ligament that is stable (functional) and one that is rotationally unstable (nonfunctional).

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TABLE I Standard Nomenclature of the American Medical Association for Athletic Injuries

Type of Sprain	Definition
1st-degree	Mild sprain (minor tear of ligament fibers) caused by direct or indirect trauma to the knee joint. The clinical presentation is characterized by pain with mild disability, mild point tenderness, no abnormal motion, little or no swelling, minimal hemorrhage, and minimal functional loss
2nd-degree	Moderate sprain (partial tear of the ligament) caused by direct or indirect trauma to the knee joint. The clinical presentation is characterized by pain with moderate disability, point tenderness, slight-to-moderate abnormal motion, swelling, localized hemorrhage, moderate loss of function, and a tendency to recur
3rd-degree	Severe sprain (complete tear of the ligament) caused by direct or indirect trauma to the knee joint. The clinical presentation is characterized by pain, disability, marked abnormal motion, possible deformity, tenderness, swelling, hemorrhage, and persistent instability

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TABLE I Standard Nomenclature of the American Medical Association for Athletic Injuries

Type of Sprain	Definition
1st-degree	Mild sprain (minor tear of ligament fibers) caused by direct or indirect trauma to the knee joint. The clinical presentation is characterized by pain with mild disability, mild point tenderness, no abnormal motion, little or no swelling, minimal hemorrhage, and minimal functional loss
2nd-degree	Moderate sprain (partial tear of the ligament) caused by direct or indirect trauma to the knee joint. The clinical presentation is characterized by pain with moderate disability, point tenderness, slight-to-moderate abnormal motion, swelling, localized hemorrhage, moderate loss of function, and a tendency to recur
3rd-degree	Severe sprain (complete tear of the ligament) caused by direct or indirect trauma to the knee joint. The clinical presentation is characterized by pain, disability, marked abnormal motion, possible deformity, tenderness, swelling, hemorrhage, and persistent instability

Sudden deceleration and rotational maneuvers place the anterior cruciate ligament at risk for injury²². Most partial injuries of the anterior cruciate ligament result from noncontact athletic activity. The mechanism of injury involves the femur being externally rotated on a fixed lower extremity or the tibia being internally rotated relative to the femur as a valgus moment is applied to the knee joint^22,44. Like patients sustaining a complete tear of the anterior cruciate ligament, those sustaining a partial injury may experience a tearing sensation or a pop, difficulty bearing weight, a sensation of instability ("giving out"), and an inability to continue activity^9,22,30. The knee pain is usually poorly localized. A hemarthrosis may develop, but its absence does not preclude an anterior cruciate ligament injury^5,8. The partially torn ligament may act as a mechanical block preventing full knee extension^1,45. In other words, a mechanical obstruction occurs as the torn segment of the ligament becomes trapped between the lateral femoral condyle and the lateral tibial plateau. Other causes of obstruction include displaced meniscal tears or loose osteochondral fragments²⁴.

The clinical diagnosis of a partial tear of the anterior cruciate ligament is based on the degree of laxity detected on physical examination¹⁷. Commonly, the increased anterior translation is defined by quantifying the amount that it is increased above a certain threshold and comparing it with that of the uninjured knee^46,47. The anterior drawer, Lachman, pivot shift, and arthrometer (e.g., KT-1000) tests are the most common methods used to make this assessment. Even though the anterior drawer test is commonly described as a means with which to assess anterior cruciate ligament laxity, it can be an unreliable measure of anterior knee laxity^41,48. When a patient has an acute injury, hamstring spasms and flexion of the knee to 90° cause pain that makes it difficult to perform an accurate examination. Secondary stabilizers of the knee (the osseous contour of the joint, the posterior meniscal horns, and the medial collateral ligament) may also interfere with the ability to obtain a true assessment of anterior knee translation. Usually, the result of the anterior drawer test is abnormal when secondary restraints are also injured. For these reasons, the Lachman test is more sensitive than the anterior drawer test for detecting anterior cruciate ligament insufficiency. Furthermore, biomechanical studies have revealed that there is greater strain in the anterior cruciate ligament at 30° of flexion than there is at 90° of flexion^14,17,30. Sectioning of the anterior cruciate ligament also results in more anterior translation at 30° of flexion than at 90°, but in order to prevent secondary stabilizers from interfering with the examination the tibia must be in neutral rotation during the test.

Some data from cadaver studies suggest that substantial injury to the anterior cruciate ligament must occur before it is possible to detect abnormal anterior laxity on physical examination¹⁷. These studies indicate that it may be difficult to distinguish a partial tear of the anterior cruciate ligament from an intact anterior cruciate ligament^17,24,49. According to Hole et al., partial injuries cannot be reliably identified during a clinical examination even when as much as 75% of the ligament is torn¹⁷. More specifically, they found significant increases (p < 0.05) in translation only after sectioning both the posterolateral bundle and half of the anteromedial bundle and after complete sectioning of the anterior cruciate ligament. In the same study, only two of eighteen examinations led to the correct identification of a partially cut anterior cruciate ligament when the posterolateral bundle had been sectioned. These results indicate that either the posterolateral bundle is dominant with regard to restraining anterior translation or 50% of the anterior cruciate ligament is enough to maintain normal findings on physical examination¹⁷. In another cadaver study, Lintner et al. found that it was not possible to detect differences between an anterior cruciate ligament in which the anteromedial bundle had been sectioned and an intact anterior cruciate ligament with a clinical examination and testing with a KT-1000 arthrometer²⁴. According to Fritschy et al., some fibers remain intact after partial tearing of the anterior cruciate ligament and, therefore, there is no initial anterior laxity of the knee, which makes clinical diagnosis difficult⁴⁹. This evidence suggests that, in some cases, a single-bundle tear may not be detectable on physical examination²⁴. In any case, an asymmetric result on the Lachman test is indicative of an anterior cruciate ligament injury that requires further investigation.

A positive pivot-shift test represents the subluxation-reduction phenomenon associated with a knee in which the anterior cruciate ligament is functionally deficient. The tibia is subluxated relative to the femur in extension and is reduced in flexion. More specifically, when the knee has a deficient anterior cruciate ligament, the femur is subluxated relative to the tibia by gravity and the iliotibial band lies anterior to the instant center of rotation of the knee. As the knee is flexed to approximately 20° to 25°, the pivot shift phenomenon occurs as the knee reduces⁴². The test result is graded as 1+ (glide), 2+ (jump), or 3+ (transient lock). This test is better than the anterior drawer or Lachman test for defining anterior cruciate ligament insufficiency, and a positive result is considered indicative of anterior cruciate ligament deficiency. It is especially useful for determining if anterior laxity will be clinically relevant^37,41,42. Therefore, the pivot shift test is the most helpful for defining the extent of an injury to the anterior cruciate ligament, especially when it is performed with the patient under anesthesia.

In many studies in the orthopaedic literature, the authors used a low-grade (grade-1+) pivot shift in their clinical definition of a partially torn, yet functional, anterior cruciate ligament^9,38,39. However, one of us (B.R.B. Jr.) and colleagues suggested that a positive pivot-shift test with the patient under anesthesia is indicative of an anterior cruciate ligament-deficient knee and, therefore, such a knee should be treated as if it has a complete injury⁴³. According to this definition, patients with a partial tear of the anterior cruciate ligament may have increased anterior translation with asymmetric results on Lachman or arthrometer testing but have a negative pivot-shift result under anesthesia. On the other hand, patients with a completely torn or partially torn nonfunctional anterior cruciate ligament have increased anterior translation on Lachman or arthrometer testing and a positive pivot-shift test (=1+).

Arthrometric Testing

With arthrometer testing at 30° of knee flexion and application of a 20-lb (89-N) force, anterior cruciate ligament deficiency is defined as a maximum side-to-side difference of >3 mm, a maximum manual translation of >10 mm, or a compliance index of >2 mm⁴³. On the basis of his experience, the senior author (B.R.B. Jr.) believes that a low-grade KT-1000 arthrometer measurement (in comparison with that of the uninjured knee) suggests, but is not diagnostic of, a partial tear of the anterior cruciate ligament. For example, a side-to-side difference of 6 mm would be consistent with a complete tear of the anterior cruciate ligament whereas a side-to-side difference of 3 mm could suggest a partial injury, but only if the pivot shift was negative. In our experience, it has been unusual to find a side-to-side difference of >3 mm in a patient with a partial injury of the anterior cruciate ligament; conversely, it should be noted that patients who have anterior cruciate ligament deficiency (with a positive pivot shift) may have a <3-mm side-to-side difference. Some studies have suggested that it is difficult to make this diagnosis with use of an arthrometer. For example, in a cadaver study, Lintner et al. revealed that partial tears of the anterior cruciate ligament were associated with a mean increase in sagittal laxity of 1.3 mm²⁴. In order to diagnose a partial tear of the anterior cruciate ligament on the basis of this criterion, clinicians would need to distinguish between a 1.3-mm increase and a 3-mm increase in translation. This small difference is very difficult to detect with current clinical methods^24,50.

Few authors of clinical studies on partial tears of the anterior cruciate ligament have reported the use of arthrometer testing as an adjunct to define pathologic anterior translation. In the studies that have included arthrometric results, the majority of patients with a partial tear of the anterior cruciate ligament have had an arthrometric measurement of =3 mm. A >3-mm difference, compared with the uninjured knee, was used as the standard for defining pathologic laxity in several studies. Noyes et al. reported that a side-to-side difference in laxity of >5 mm was a poor prognostic sign, as most patients with a partial tear of the anterior cruciate ligament had a difference that was below this threshold⁹. In fact, 61% (fourteen) of their twenty-three patients had a <3-mm increase in laxity. The average side-to-side difference (and standard deviation) for knees classified as having a functional anterior cruciate ligament was 0.9 ± 1.8 mm. Those described as having a deficient anterior cruciate ligament at the time of follow-up had an average difference of 5.6 ± 1.6 mm. Bak et al. reported that thirty of thirty-four patients had a mean side-to-side difference of =3 mm⁵¹. Fruensgaard and Johannsen reported an average difference of 2.8 mm; eighteen patients with moderate-to-severe instability had anterior laxity of 4 to 12 mm¹¹. Exact measurements with reference to specific patients were not included in these results. Fritschy et al. reported that sagittal laxity never exceeded 3 mm in any of the forty-three patients in their study⁴⁹. Messner and Maletius reported that, at a twelve-year follow-up visit, 91% (twenty) of twenty-two patients had an increase in sagittal laxity that measured =3 mm³⁹. This result was unchanged at the long-term follow-up visit at twenty years after the initial injury³⁹. Sommerlath et al. reported that, of twenty-two patients available for arthrometric testing, 86% (nineteen) had an increase in laxity of <3 mm⁵².

Overall, arthrometric testing is a useful adjunct in the assessment of sagittal laxity of the knee, but it may not be sensitive enough to identify partial tears of the anterior cruciate ligament unless there is substantial damage to the ligament. Clinical studies have shown that most knees with a partial tear of the anterior cruciate ligament do not reveal pathologic laxity on arthrometric testing. More than 3 mm of anterior translation on arthrometric testing suggests anterior cruciate ligament deficiency. Less than 3 mm does not definitely correlate with a partial tear of the anterior cruciate ligament, and it does not give any meaningful information regarding the quality or functional capacity of the intact tissue.

Magnetic Resonance Imaging

In general, the findings on a magnetic resonance imaging scan may suggest a partial tear of the anterior cruciate ligament, but they cannot be used to make a definitive diagnosis. Magnetic resonance imaging has been reported to have a low sensitivity and specificity for diagnosing partial tears of the anterior cruciate ligament because of the variability in the degree of ligament disruption and in its appearance on the scans^10,18,40. The criterion for defining a partial tear of the anterior cruciate ligament on the basis of magnetic resonance imaging has been described as the presence of high signal intensity within the anterior cruciate ligament, which may have a wavy course and/or focal thinning but maintains its continuity and orientation^53,54. In most cases, sagittal images are used to assess signs of anterior cruciate ligament injury and coronal and axial images are used to confirm these findings^54,55. Because of its oblique course, however, the anterior cruciate ligament is not well visualized throughout its entire length on standard orthogonal views. Additional parasagittal and paracoronal (oblique) planes can be used to improve visualization⁵³. Proponents of these views suggest that they allow a more straightforward assessment of the thickness, signal intensity, continuity, and contour of the anterior cruciate ligament⁵³. However, the addition of oblique views is of marginal value in terms of improving the accuracy of diagnosis of partial tears of the anterior cruciate ligament compared with that possible with standard orthogonal views^40,53,56. Nevertheless, a 3-T magnetic resonance imaging protocol is currently being studied in the hopes of improving the capacity to image isolated bundles of the anterior cruciate ligament and the accuracy of diagnosis of partial tears of the anterior cruciate ligament^57,58. With use of this technology, oblique imaging planes are combined with an increase in the signal-to-noise ratio and resolution to allow a more accurate assessment of both bundles of the anterior cruciate ligament⁵⁸.

It is important to note that half or more of detected partial ruptures of the anterior cruciate ligament are indistinguishable from complete ruptures on magnetic resonance imaging¹⁰. Primary signs (those involving the anterior cruciate ligament itself) and secondary signs (associated abnormalities) are used to determine the extent of injury. Primary signs include loss of ligament fiber continuity, mass effect, increased signal intensity within the ligament, and abnormal ligament contour. Secondary signs include osteochondral lesions, excessive buckling of the posterior cruciate ligament, anterior tibial translation, and a deep lateral femoral notch⁵⁴. The primary signs are present in only 33% (twenty-nine of eighty-eight) to 43% (thirty-eight of eighty-eight) of patients with a partial rupture of the anterior cruciate ligament⁹. Furthermore, many secondary signs do not significantly improve the detection of partial ruptures¹⁰. In a study performed by Umans et al., magnetic resonance imaging was found to have a 55% sensitivity and a 75% specificity for the diagnosis of partial tears of the anterior cruciate ligament when the diagnosis was made on the basis of the presence of one or more of the following criteria: abnormal intrasubstance signal intensity within intact ligament fibers, bowing or undulation of otherwise intact fibers, nonvisualization on the T1-weighted image with intact fibers seen on inverse-recovery or T2-weighted pulse sequences, and the absence of secondary signs¹⁸. Even though these criteria are helpful for defining the injury, more research is needed to improve the ability of magnetic resonance imaging to distinguish partial from complete tears of the anterior cruciate ligament. Until this goal is achieved, magnetic resonance imaging should be used only as an adjunct to other clinical modalities to diagnose partial tears of the anterior cruciate ligament.

Arthroscopy

Since the clinical assessment of a partial tear of the anterior cruciate ligament is difficult, many surgeons use arthroscopy to define the extent of the injury^{1,3,5,7-9,24,59}. Noyes et al. postulated that arthroscopic assessment of a partial tear of the anterior cruciate ligament is the best way to estimate the amount of gross tearing—i.e., whether it involves one-fourth, one-half, or three-fourths of the anterior cruciate ligament⁹. However, accurate grading of the percentage of torn anterior cruciate ligament tissue is difficult for several reasons. Anterior cruciate ligament tissue that appears to be intact may mask a substantial injury. The ligament tissue may undergo plastic deformation and appear intact when it is lax and nonfunctional. In some situations, the synovial sheath may be preserved and the ligament may appear minimally injured. Many injuries, particularly among skiers, occur proximally near the femoral origin and, unless it is carefully assessed for fiber orientation, most of the distal part of the ligament may appear normal. A completely torn anterior cruciate ligament may also be attached to the surrounding intercondylar notch or posterior cruciate ligament by scar tissue, leaving a normally appearing strut (a so-called vertical strut) of tissue that may be misinterpreted as an intact or partially torn anterior cruciate ligament⁶⁰.

Some surgeons have found a correlation between the amount of injury visualized at the time of arthroscopy and the clinical outcome⁹, but others have not^11,52,61. This discrepancy reflects the imprecision of the arthroscopic grading of anterior cruciate ligament tears and indicates that a torn but functional ligament cannot always be identified with arthroscopy. Indeed, even with the aid of arthroscopy, it is difficult to determine the competence of an injured anterior cruciate ligament (Fig. 1). Surgeons should use arthroscopy to help define the injury but not as the sole method of diagnosis. The use of arthroscopy should be considered to determine unequivocally that an anterior cruciate ligament that is suspected to have a partial injury is not, in fact, functionally deficient (with a positive pivot shift). Candidates for arthroscopic evaluation should be identified on the basis of the history, findings on magnetic resonance imaging, and findings on physical examination, including an examination with the patient under anesthesia. The combination of these modalities provides the necessary information to make decisions about appropriate treatment, including whether operative intervention is necessary.

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Fig. 1: Arthroscopic appearance of a vertical anterior cruciate ligament strut in the left knee. The anterior cruciate ligament tissue looks relatively normal, albeit vertically oriented. On probing, the lateral wall was found to be empty (not shown). The patient had a 2+ pivot shift. This is an example of a situation in which the appearance of the anterior cruciate ligament might have led the surgeon to believe that the patient had a partial injury.

Treatment

Treatment must be customized for a patient who sustains a true partial injury of the anterior cruciate ligament. Development of a treatment strategy requires consideration of symptoms, the findings on the physical examination, the percentage of the ligament that is injured, associated injuries, the time interval from the initial injury, job requirements, athletic activity level, and goals for continued activity.

We believe that identifying patients as being at low or high risk for progression to anterior cruciate ligament deficiency is a critical step in developing the treatment strategy. Patients identified as being at low risk (those with a low-demand lifestyle) usually have minimal symptoms and a negative pivot-shift test. Their symptoms are unlikely to progress, and therefore they should be treated nonoperatively. Patients in the high-risk group (those with a high-demand lifestyle) can be managed initially with a nonoperative program, but the risk of reinjury should be explained, especially if the patient anticipates that he or she will participate in activities that place a high amount of stress across the knee joint. When a patient is at high risk, substantial symptomatic knee joint instability (a positive pivot-shift test) may develop and, as a result, the individual may become a candidate for operative treatment.

Nonoperative Management

Nonoperative management may include a short period of immobilization, bracing, a progressive rehabilitation program, and regular follow-up evaluations^{3,4,9,11,13,32,38,46,49,51,52,62}. Various types of immobilization, including casts, removable splints, and soft dressings, have been described for the treatment of partial tears of the anterior cruciate ligament. If the treating surgeon wants to immobilize and protect the injured knee, a hinged knee brace seems most versatile, as it provides protection when the patient walks during the acute phase of the injury and it can be used during range-of-motion exercises. We do not use immobilization or bracing for patients with a partial tear of the anterior cruciate ligament. Instead, we initiate a rehabilitation program as soon as possible. An early range of motion is the goal in order to prevent arthrofibrosis. The patient progresses from non-weight-bearing or partial weight-bearing to weight-bearing as tolerated as the acute phase of the injury resolves. The principles of rehabilitation for patients with a partial tear of the anterior cruciate ligament follow the guidelines for those with a complete anterior cruciate ligament tear. Strength tests and functional performance tests should be used to decide when athletes can return to a preinjury level of activity⁶³. Overall, the rehabilitation program should consist of range-of-motion exercises, lower-extremity muscle strengthening, core strengthening, cardiovascular endurance training, perturbation training, and sport-specific skill training⁶⁴.

There are no clear guidelines regarding the role of bracing for patients with a partial tear of the anterior cruciate ligament. However, given the interstitial damage that occurs in the ligament at the time of injury, it may be prudent for the patient to wear a brace during the early rehabilitation period. Even though the correct duration for which the anterior cruciate ligament should be protected before the athlete returns to sports activity is unknown, he or she should anticipate returning to full activity approximately three months after an isolated partial tear of the anterior cruciate ligament. There is no evidence to suggest that bracing prevents the progression of a partial anterior cruciate ligament tear to a complete tear as a patient resumes sports activity. Evaluating patients at regular intervals is important to monitor their progress with rehabilitation and their return to sports. In this way, the treatment can be tailored to achieve an acceptable functional outcome⁵¹.

Nonoperative management of partial tears of the anterior cruciate ligament has reportedly resulted in successful clinical outcomes in some clinical series. These results were defined as good-to-excellent functional outcomes, maintenance of or some reduction in activity level, and a minimal increase in laxity^7,9,37,52. Others have suggested that partial tears of the anterior cruciate ligament are functionally equivalent to complete tears and nonoperative management has poor results^17,19,37. These differences are most likely related to variations in the definition of a partial injury of the anterior cruciate ligament. When symptomatic knee instability develops, the patient is a candidate for operative treatment.

Operative Treatment

Operative management of partial tears of the anterior cruciate ligament begins with an examination with the patient under anesthesia to confirm the diagnosis. Since the accuracy of the physical examination is often limited by patient guarding and muscle spasms, use of anesthesia is essential. While the patient is awake, the sensitivity of a pivot shift test is only 24%, whereas it improves to 92% when the patient is under anesthesia⁴². The result of the pivot shift test with the patient under anesthesia is the most important diagnostic factor in the assessment of the functional status of the native anterior cruciate ligament^41,42. A positive pivot-shift test indicates a functional loss of the rotational stability imparted by the anterior cruciate ligament, which is not demonstrated by the measurement of anterior laxity alone⁴². A positive pivot-shift test also eliminates the need to visually estimate the percentage of the anterior cruciate ligament fibers that are torn.

We believe that, if the pivot shift test is negative when it is performed during the clinical examination in the surgeon's office, but the surgeon suspects an anterior cruciate ligament injury, then the patient should be scheduled for an examination under anesthesia. If the pivot shift test is negative while the patient is under anesthesia, then the surgeon might wish to proceed with diagnostic arthroscopy. The purpose of the diagnostic arthroscopy is to rule out injuries that can prevent one from obtaining an accurate result of the pivot shift test, such as displaced anterior cruciate ligament tissue, a displaced meniscus, or osteochondral fragments. Although magnetic resonance imaging is an excellent diagnostic test, knee injuries (meniscal tears, osteochondral injuries, and partial tears of the anterior cruciate ligament) can be missed during interpretation of the imaging study. For this reason, it is our preference to proceed with arthroscopy if we have a high suspicion that we are not obtaining a reliable result during an examination with anesthesia because of intra-articular pathology. If any of these injuries are present, then they should be addressed (e.g., with resection of displaced anterior cruciate ligament tissue), and then the pivot shift test should be repeated. If the pivot shift test is negative, then the patient should be treated nonoperatively. If the pivot shift test is positive, then the patient should be treated with an anterior cruciate ligament reconstruction.

The options for operative management of a partial tear of the anterior cruciate ligament have included thermal modification, single-bundle repair, and anterior cruciate ligament reconstruction. Techniques for these procedures are described elsewhere^13,36,65-70.

Thermal Modification

Thermal sources of energy used during arthroscopic knee procedures include lasers and radiofrequency devices (monopolar and bipolar). The radiofrequency probes are used to ablate tissue and reduce laxity. We are not aware of any long-term human trials demonstrating the efficacy and safety of these procedures on the cruciate ligaments of the knee. To our knowledge, all of the published studies provide less-than-sufficient evidence to support clinical recommendations. Some surgeons have had success with this technique and recommend it with cautious optimism^36,65,71. Proponents suggest that, with cautious application, monopolar radiofrequency is a possible treatment option for partial tears of the anterior cruciate ligament and a reasonable alternative to anterior cruciate ligament grafting in selected patients^36,65,71. Others, however, have not had good results and do not share this opinion^66-68.

In vivo, in vitro, and laboratory studies of clinical specimens have shown that the tensile stiffness of collagen is diminished after thermal treatment^72-74. These data were obtained, in large part, from studies performed on capsular tissue of the shoulder joint, not the cruciate ligaments of the knee. Nevertheless, applying the concept of inadequate tensile strength to the cruciate ligaments allows one to understand the clinical sequelae of thermal treatment of those ligaments. The biomechanical characteristics of thermally treated tissues and the subsequent healing of those tissues have not been well defined⁶⁶. Overall, more research is needed to confirm the efficacy of this technique before it can be recommended for treatment of partial tears of the anterior cruciate ligament. At this time, thermal modification is not recommended for the treatment of confirmed partial tears of the anterior cruciate ligament.

Single-Bundle Repair and Reconstruction of the Anterior Cruciate Ligament

Single-bundle repair with use of a suture technique with femoral tunnels is rarely performed for the treatment of partial tears of the anterior cruciate ligament^13,52. Sommerlath et al. reported retrospectively on three patients who received this treatment⁵². Twelve years after the operation, two of the three patients had an increase in knee laxity of =3 mm, as compared with that of the uninjured knee, and a decrease in activity level, as compared with preoperative function, but they had the same average knee function score as the rest of the patients in the study. The clinical outcome for the third patient was not discussed. Although repairing an isolated anterior cruciate ligament bundle tear is a viable treatment option, the most common operative treatment for patients with symptomatic instability after a partial tear of the anterior cruciate ligament is reconstruction.

Single-bundle reconstruction is performed by selectively reconstructing the anteromedial or posterolateral bundle. We believe that the term "partial" or "isolated bundle" injury does not reflect microinjury within the continuum of the anterior cruciate ligament fibers. Our primary goal of anterior cruciate ligament reconstruction is to eliminate the pivot shift phenomenon. A recent study on cadavers suggested that part of both bundles can be reconstructed with use of this technique⁷⁵.

If a patient has a single-bundle injury with a negative pivot shift and marginally increased anterior knee laxity, then (on the basis of our definition of a partial tear of the anterior cruciate ligament and our clinical experience) we treat the patient nonoperatively. If the injury resulted in a positive pivot shift, then we interpret this finding as a loss of functional stability and discuss operative treatment with the patient.

Overall, the clinical outcomes of anterior cruciate ligament reconstruction have not been reported to differ substantially between patients with a partial tear and those with a complete tear. Therefore, when nonoperative management results in clinical failure, we believe that anterior cruciate ligament reconstruction is the treatment of choice to restore knee stability, to avoid injury to the menisci and articular cartilage, and to enable the patient to return to a desired level of activity.

Clinical Outcomes

Published studies on partial tears of the anterior cruciate ligament have had several limitations. For example, most have provided Level-IV evidence and have lacked consistent clinical evaluation, a description of power analysis, an adequate sample size, and confidence intervals to support statements about statistically significant and clinically relevant results. Additionally, these studies have included either only short-term follow-up or a limited number of patients. These limitations have contributed to the inconsistencies in the definition of partial tears of the anterior cruciate ligament and in the clinical evaluation of patients. Indeed, the small number of patients in each study combined with the inadequate statistical analysis makes the interpretation and comparison of results difficult.

A table in the Appendix contains a list of studies on the nonoperative and operative treatment of partial tears of the anterior cruciate ligament. The average age of the patients in the studies was twenty years (range, fourteen to fifty-five years). The diagnosis of a partial tear of the anterior cruciate ligament was determined in most studies on the basis of both an examination with the patient under anesthesia and arthroscopy^{4,9,11,36-39,46,47}. Since the substantial amount of variability in the clinical assessments used to define patient outcome and the lack of rigorous statistical analysis in some studies prevented meaningful comparisons, the clinical assessment scores are not listed in the table^{3,4,7,9,11,13,32,37-39,46,47,49,51,76-78}. The factors that were most relevant to patient outcome in the reported studies included the average time to follow-up, the percentage of patients who were seen with insufficiency of the anterior cruciate ligament at the time of follow-up, the percentage of patients who were able to return to their previous level of activity after nonoperative management, and the percentage for whom nonoperative management failed and who went on to have an anterior cruciate ligament reconstruction.

Despite reports that the course and long-term results following nonoperative management of partial tears of the anterior cruciate ligament are favorable^{9,11,32,46,52}, untreated partial tears may proceed to complete rupture. The clinical outcome of partial tears of the anterior cruciate ligament is defined by the risk of progression to complete deficiency of the anterior cruciate ligament, the alteration in activity level, the influence of concomitant knee injuries (to the menisci, ligaments, and articular cartilage), and the risk of subsequent degenerative joint disease. Understanding these factors is essential not only to developing a treatment plan, but also to counseling patients and defining their expectations. Another confounding variable is that the force placed on the knee during the performance of the pivot shift test with the patient under anesthesia does not compare with the forces created by cutting, jumping, and landing.

Natural History

The natural history of partial tears of the anterior cruciate ligament is not well defined or consistent^4,36. The variety of criteria that have been used to establish the diagnosis complicates the analysis of the natural history. Furthermore, in most studies, the authors could not reliably assess the natural history because patients frequently continued to participate in activities and, as a result, sustained a recurrent injury^8,32. Clinical studies suggest that partial tears of the anterior cruciate ligament may follow a benign course and there may be a favorable evolution to successful function^{3,7,9,32,47,52,76-78}. However, published reports also suggest that a substantial number of partial tears progress to complete deficiency of the anterior cruciate ligament and symptomatic knee laxity^{4,9,11,12,38,49,65}. The prevalence of subsequent insufficiency of the anterior cruciate ligament has been reported to range from 11% (one of nine)⁷ to 62% (eighteen of twenty-nine)⁷⁶ in cases of nonoperatively treated partial tears of the anterior cruciate ligament in adult patients followed for 1.5 to twenty years after the initial injury^{4,9,11,32,37-39,46,49,51,52,78}.

According to Noyes et al., three factors have a significant relationship (p < 0.01) with progression to anterior cruciate ligament deficiency: (1) the amount of tearing of the anterior cruciate ligament seen at the initial arthroscopy; (2) an increase in anterior translation, compared with the contralateral side, at the initial examination; and (3) the subsequent occurrence of a giving-way reinjury⁹. Furthermore, partial tears involving 50% or 75% of the anterior cruciate ligament progress more frequently than do tears involving <25% of the ligament⁹. Kocher et al. used univariate analysis in a study of patients with a partial tear of the anterior cruciate ligament to identify factors that were significantly associated with the need for subsequent reconstruction of the anterior cruciate ligament³⁷. These factors included tears involving >50% of the ligament (p = 0.02), tears of the posterolateral bundle (p = 0.02), a grade-B (glide) pivot shift, a chronological age of more than fourteen years old, and a skeletal age of more than fourteen years old. There were no significant associations between the need for subsequent reconstruction of the anterior cruciate ligament and sex, skeletal maturity, the Lachman test result, the involved side, or a concomitant meniscal injury. In the same study, a multivariate analysis identified only tears involving >50% of the ligament and posterolateral bundle tears as significant independent predictors of subsequent reconstruction of the anterior cruciate ligament. Survivorship analysis identified older skeletal age, a tear involving >50% of the ligament, and posterolateral bundle tears as significant time-dependent risk factors for subsequent reconstruction of the anterior cruciate ligament (p = 0.01).

The relationship between the percentage of ligament damage and the prognosis is controversial. According to some studies, the percentage of ligament damage can predict the subsequent development of instability^9,37. For example, Noyes et al. noted a 50% chance of instability and a 75% chance of additional knee injury when 50% of the anterior cruciate ligament tissue was found to be disrupted at the time of arthroscopy⁹. This finding is supported by the results in the study by Kocher et al., who reported that tearing of >50% of the anterior cruciate ligament is a significant risk factor for progression³⁷. However, the results of other studies suggest that there is no correlation between the percentage of ligament injury and the clinical outcome^4,38. Regardless of which belief is correct, recurrent injury and progression to complete anterior cruciate ligament deficiency limit the ability to perform demanding activities, such as twisting, cutting, pivoting, and jumping⁹. We maintain that one cannot accurately consider these factors unless the pivot shift test is negative; a positive pivot-shift test, regardless of the grade, defines a functionally deficient anterior cruciate ligament. The likelihood of recurrent injury and the need for reconstruction clearly differ between partial and complete injuries.

Level of Function

Despite acceptable knee function and stability at the time of early follow-up of patients with a partial tear of the anterior cruciate ligament, the general trend is toward a reduction in the level of activity of patients who do not undergo reconstruction^{11,39,46,51,52}. In fact, as few as 18% of patients (ten of fifty-six) with a partial tear of the anterior cruciate ligament return to their preinjury level of activity⁵¹. Even patients who have no clinical loss of function may have to reduce their sports activity^{4,9,11,38,39,46,51,52}. Depending on the extent of injury, instability may develop not only with athletic activity, but also with activities of daily living³⁸. The desired level of activity may influence the prognosis to the same degree as does the extent or the location of the tear⁵¹. The results of previous studies have shown that sports involving cutting or pivoting can be resumed after a partial tear of the anterior cruciate ligament but these activities place the knee at increased risk for recurrent injury^9,51. Alternatively, the prognosis in terms of knee function is good to excellent if activity modification includes avoidance of these stressful activities^9,51.

Most patients who report favorable function despite measurable static anterior instability describe a diminished activity level and avoidance of strenuous sports⁹. Overall, a substantial number of partial tears progress to complete rupture of the anterior cruciate ligament, and the majority of patients are unable to return to their preinjury activity level^{4,9,46,49,51,52}. In most of these cases, there is a downgrade in activity level from competitive team sports to individual sports and physical fitness exercises³⁹.

Associated Injuries

Other structures of the knee can be injured at the time of partial tears of the anterior cruciate ligament. These structures include the menisci (more often the medial meniscus than the lateral meniscus), the collateral ligaments (more often the medial than the lateral), and the articular cartilage^{3,4,7,9,32,37-39,46,47,52,78}. Despite injury to these surrounding structures, there is no reported difference in the clinical outcome and the need for reconstruction between patients with associated injuries and those with an isolated partial tear of the anterior cruciate ligament^{3,4,7,9,32,37,39}.

Risk of Subsequent Degenerative Joint Disease

There are limited data on the development of posttraumatic degenerative joint disease after partial tears of the anterior cruciate ligament. In a retrospective review of nonoperatively treated partial tears of the anterior cruciate ligament, Kannus and Jarvinen reported that posttraumatic degenerative joint disease developed in 15% (six) of forty-one patients at an average of eight years after a partial tear of the anterior cruciate ligament⁴⁷. None of these patients had instability that was severe enough to warrant reconstruction, but four of them required meniscectomy. In their series of partial tears of the anterior cruciate ligament, Messner and Maletius reported that only 55% (twelve) of twenty-two patients were available for radiographic follow-up at twelve and twenty years after injury³⁹. Two of the twelve patients who had had normal radiographic findings at twelve years progressed to having minimal signs of osteoarthritic changes at twenty years, whereas three of the twelve had high-grade osteoarthritis at twenty years. Because of the limited number of studies focusing on this issue and their inadequate power, no definitive conclusions can be drawn regarding the relationship between partial tears of the anterior cruciate ligament and the development of degenerative joint disease. However, instability initiated by a partial tear of the anterior cruciate ligament and a concomitant meniscal tear requiring meniscectomy or chondral injury places the patient at increased risk for the development of posttraumatic degenerative joint disease.

Overview

A thorough clinical assessment is required to determine the extent of an anterior cruciate ligament injury. The pivot shift test is the most important factor in defining the injury. Physical examination, measurement with a KT-1000 arthrometer, and magnetic resonance imaging may result in an underestimation of the degree of injury, leading the physician to conclude that the patient has only a partial tear. If a partial tear of the anterior cruciate ligament is suspected, then additional tests should be directed toward differentiating between a functional anterior cruciate ligament and a functionally deficient anterior cruciate ligament. A partial tear of the anterior cruciate ligament should be defined functionally and not structurally, to guide subsequent treatment. An examination with the patient under anesthesia and arthroscopic evaluation are indicated to achieve this goal. The pivot shift test performed with the patient under anesthesia is the most important factor in defining the extent of the anterior cruciate ligament injury. An abnormal pivot-shift result is consistent with a functionally deficient anterior cruciate ligament. The results of this test allow the surgeon to make a decision regarding whether to proceed with nonoperative management or reconstruction.

Appendix

A table listing reports of the treatment of partial tears of the anterior cruciate ligament is available with the electronic versions of this article, on our web site at (go to the article citation and click on "Supplementary Material") and on our quarterly CD/DVD (call our subscription department, at 781-449-9780, to order the CD or DVD). Image Not Available

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Topics

arthroscopy ; anterior cruciate ligament ; knee joint ; ligaments ; pivot shift test response ; anterior cruciate ligament reconstruction ; anterior cruciate ligament injury

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Michael J. DeFranco, Bernard R. BachJr.; A Comprehensive Review of Partial Anterior Cruciate Ligament Tears. The Journal of Bone & Joint Surgery. 2009 Jan;91(1):198-208.

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