Look for this and other related articles in Instructional Course Lectures, Volume 57, which will be published by the American Academy of Orthopaedic Surgeons in March 2008:"Extensor Mechanism Complications After Total Knee Arthroplasty," by Jay Patel, MD, Michael Ries, MD, and Kevin Bozic, MDInstability after total knee arthroplasty is a cause of failure and a reason for 10% to 22% of revisions1-5. Successful outcomes can be obtained in many of these cases, but without identifying the cause of instability, the surgeon risks repeating the mistakes that led to the instability after the initial total knee arthroplasty2,3,6. As Vince et al.6 stated, "the patient's report of instability is not a diagnosis", and particular care should be given to confirming the diagnosis and to understanding the causes.
Look for this and other related articles in Instructional Course Lectures, Volume 57, which will be published by the American Academy of Orthopaedic Surgeons in March 2008:"Extensor Mechanism Complications After Total Knee Arthroplasty," by Jay Patel, MD, Michael Ries, MD, and Kevin Bozic, MD
"Extensor Mechanism Complications After Total Knee Arthroplasty," by Jay Patel, MD, Michael Ries, MD, and Kevin Bozic, MD
The first step in confirming the diagnosis and understanding the causes is clinical and includes recording an accurate and complete history, including the reason for the original knee replacement, the presence of any preoperative deformity or contracture, previous knee procedures, the specifics of the operative technique for the knee replacement, the type of prosthesis that was used, the postoperative rehabilitation program, and whether the patient sustained any trauma to the knee after the surgery1,2,6. Specific patient-related risk factors are a large surgical correction including an aggressive ligament release, general or regional neuromuscular pathology, hip or foot deformities, and clinical obesity6. The postoperative symptoms should be clarified, and a complete physical examination must be performed1,2,6. Specific attention should be paid to the knee, with the examiner noting varus-valgus laxity in extension, in 30° of flexion, and in 90° of flexion. Anteroposterior laxity should be tested as well. Laxity in flexion, so-called flexion laxity, is often most evident when an anterior or posterior drawer test is performed with the patient sitting and the knee flexed 90°7. Analysis of a complete set of radiographs is necessary and should include measurement of the mechanical and anatomical axes on anteroposterior short and full-length weight-bearing radiographs as well as measurement of implant position on lateral radiographs made with the knee in full extension and in maximum knee flexion1,2,6.
After this analysis, the type of instability can be identified. There are three types of instability after a total knee arthroplasty: extension instability, flexion instability, and genu recurvatum. Instability is accentuated by errors in component orientation or overall limb-alignment problems1-6. The goal of this Instructional Course Lecture is to provide an overview of the causes and treatments of each type of instability.
Flexion instability is seen most often in patients in whom the total knee prosthesis is well aligned axially and well fixed. Historically, this problem has been underdiagnosed in patients with a cruciate-retaining knee implant. The laxity is due to inadequate filling of the flexion space with the implant or disruption of the posterior cruciate ligament. The manifestations of flexion instability range from a vague sense of instability to frank dislocation. This variability depends in part on whether a posterior stabilized or cruciate-retaining implant is in place. Assessment of the knee in 90° of flexion should be part of the routine physical examination of any patient with pain at the site of a total knee arthroplasty, regardless of whether the prosthesis is cruciate-retaining, cruciate-sacrificing, or cruciate-substituting, as this may be the only way to recognize flexion laxity.
Dislocation after a posterior stabilized total knee arthroplasty is a rare but dramatic and disconcerting problem for patient and surgeon alike20. Most current posterior stabilized designs have increased the so-called jump distance that is needed for the cam to ride over the post before dislocating, and dislocation rates are now much lower than 0.5%. However, exceeding the so-called jump distance results in a posterior dislocation of the tibia on the femur (Fig. 5), which often requires closed reduction with the patient under anesthesia. Once reduced, the knee typically functions well but is prone to subsequent dislocation. The most common activity that leads to a dislocation is marked knee flexion plus a varus stress (placing the ankle of the operatively treated limb on the contralateral knee to put on a sock or shoe)20. A posterior stabilized knee prosthesis can resist posterior translation because of the tibial cam, but it does not resist a combination of varus or valgus stress and posterior translation. The standard posterior stabilized knee prosthesis does not provide varus-valgus constraint, and most often a loose flexion gap associated with laxity of a collateral ligament (most often the lateral collateral ligament) is the cause of flexion instability of posterior stabilized knee replacements. At-risk patients include those who had correction of a large valgus deformity, particularly if they quickly regained knee flexion with aggressive postoperative rehabilitation20. The first episode of dislocation should be treated with closed reduction, a trial of bracing, and avoidance of the activity that induced the dislocation. Recurrent dislocation should be addressed with insertion of a thicker polyethylene insert (if there is room in the extension space) or by conversion to a constrained condylar implant. The new construct should be checked in the "figure-4" position to establish that the instability has been corrected20.
A study by Schwab et al. showed that well-aligned and well-fixed posterior stabilized total knee replacements can also be symptomatically unstable in flexion without dislocating20. Their patients with this type of instability presented with a typical constellation of symptoms and physical findings, including a sense of instability without frank giving-way, difficulty ascending and descending stairs, and recurrent knee effusions. In many cases, the knee had been aspirated on one or more occasions before the diagnosis of flexion instability was made. The patients had diffuse periretinacular tenderness, especially at the site of tendinous attachments. Examination of the knee in 90° of flexion, particularly with the patient seated and his or her foot planted on the ground, was the most effective way to demonstrate the instability. Excessive anterior translation, especially if it reproduced symptoms, was indicative of flexion instability. In the study by Schwab et al., ten of 1370 revision total knee arthroplasties were done to address symptomatic flexion instability; eight of these ten cases of instability were treated successfully with revision of both the femoral and the tibial component with an emphasis on obtaining balanced flexion and extension gaps20. In this group of patients, revision total knee arthroplasty with careful attention to filling the flexion space, usually with a larger femoral component and posterior femoral modular metal augmentation, improved pain relief and stability. With the modular wedges and augments available in contemporary total knee arthroplasty revision systems, obtaining good balance of the flexion and extension spaces by upsizing the femoral component is consistently possible. The goal in these patients is to have <5 mm of anterior translation of the tibia when the knee is tested intraoperatively at 90° of flexion with the patella reduced in the femoral trochlea.
Flexion instability is an under-recognized cause of poor results after a cruciate-retaining total knee arthroplasty7. Typically, this instability occurs in knees in which the prosthesis is well aligned, well fixed, and stable to varus-valgus stress in extension7. These patients report a sense of knee instability without giving-way, recurrent swelling of the knee, and pain and tenderness about the knee7. Usually, physical examination reveals a substantial posterior sag or drawer (best seen with the patient sitting relaxed) (Figs. 6-A and 6-B), a knee effusion, and multiple areas of soft-tissue tenderness in the retinacular and pes anserinus regions.
In a previous report, two broad groups of causes for flexion instability after posterior cruciate ligament- retaining total knee arthroplasty were identified7. One is the creation of an excessive flexion gap by the surgical technical error of undersizing the femoral component or creating an excessive tibial slope. The other is a late failure of the posterior cruciate ligament21. A flat tibial liner (in the sagittal plane) offers no inherent resistance to posterior-anterior translation and may contribute to flexion instability. Similarly, posteromedial polyethylene wear functionally increases the flexion space over time and can lead to polyethylene synovitis and flexion instability.
Instability often develops early after a total knee arthroplasty in which the femoral component is undersized in the anteroposterior dimension7. Undersizing in the anteroposterior dimension can be detected by comparing a lateral radiograph of the knee made preoperatively with one made after the total knee arthroplasty. If the total knee component is undersized, the posterior condyles of the femur will appear overresected compared with those on the preoperative radiograph. This overresection results in a decrease in the so-called posterior offset of the femur, and that contributes to flexion instability and a decrease in the ultimate range of motion of the knee replacement. This undersizing is corrected with revision to a larger femoral component coupled with the use of posterior wedges. An excessive tibial slope can also lead to the total knee prosthesis being well balanced in extension but lax in flexion. Excessive slope also places the posterior cruciate ligament at risk for iatrogenic damage, which often manifests as instability early after the total knee arthroplasty. In these cases, revision total knee arthroplasty with a focus on rebalancing the flexion-extension gaps typically is most reliably achieved by conversion to a posterior stabilized implant design.
Direct iatrogenic injury to the posterior cruciate ligament at the time of the operation may be a cause of flexion instability after a cruciate-retaining total knee arthroplasty7. Alternatively, indirect iatrogenic failure of the posterior cruciate ligament can occur when the flexion space is left too tight during the surgery7,22. This makes the knee tight in flexion, and some patients will then work aggressively to improve flexion, causing the posterior cruciate ligament to rupture. Those patients can often recall a specific event when a pop or a snap occurred during vigorous rehabilitation, after which the patient usually had an immediate improvement in the range of motion. This may then be followed by progressive instability. Late instability is also occasionally observed when the posterior cruciate ligament is intrinsically weak secondary to age-related degenerative changes or following the reactivation of inflammatory disease23.
Nonoperative treatment, including quadriceps and hamstring strengthening and local modalities to address tenderness and swelling, is useful for patients with nuisance-type symptoms, but typically it has not been successful when the patient has more marked disability attributable to the flexion instability7.
Operative management with a tibial polyethylene exchange alone is one option, but poor and unpredictable results have been previously described23 following that isolated procedure. This solution does not address the underlying imbalance between the flexion and extension gaps and is therefore not recommended23. A revision to a posterior stabilized total knee arthroplasty is preferred. That procedure allows identification of the cause of instability and addresses it directly so that the knee can be balanced in flexion and extension23. At the time of revision, a larger femoral component with posterior augments is often required, and tibial slope, if it is not perpendicular to the long axis of the tibia, should be corrected to closely equalize the flexion and extension spaces. It is important to recognize that the post and cam of the posterior stabilized implant alone is not enough to stabilize these knees, so simply switching to a posterior stabilized prosthesis without carefully balancing the gaps is unlikely to be successful. In our previous series, nineteen of twenty-two knees that were revised to a posterior stabilized design, with careful intraoperative attention given to balancing the flexion and extension gaps, were satisfactorily improved7.
Instability after total knee arthroplasty is a problem that often can be avoided by identifying patients who are at risk and by paying close attention to balancing of the flexion-extension gaps at the time of the primary total knee arthroplasty. The distal femoral cut will selectively influence the extension space, the posterior femoral cut will selectively influence the extension space, and the tibial cut affects both spaces. The evaluation of an unstable total knee replacement should focus on clear identification of the etiology. In particular, all knees with pain after a total knee arthroplasty should be examined in 90° of flexion to assess for flexion instability, which has historically been underdiagnosed. Surgical treatment is largely aimed at restoring balanced flexion and extension gaps at the time of revision total knee arthroplasty. Selective use of constrained and rotating-hinge total knee arthroplasty designs is appropriate for subgroups of patients with instability problems.