The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 87, Issue 2

Scientific Articles | February 01, 2005

Can Normal Knee Kinematics Be Restored with Unicompartmental Knee Replacement?

Shantanu Patil, MD¹; Clifford hW. ColwellJr., MD¹; Kace A. Ezzet, MD¹; Darryl D. D'Lima, MD¹

¹ Orthopaedic Research Laboratories, Scripps Clinic Center for Orthopaedic Research and Education, 11025 North Torrey Pines Road, Suite 140, La Jolla, CA 92037. E-mail address for D.D. D'Lima: ddlima@scripps.edu

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In support of their research or preparation of this manuscript, one or more of the authors received grants or outside funding from DePuy, a Johnson and Johnson company, and from Alfred A. Smith and Susan Smith D. Richardson. None of the authors 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, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

Investigation performed at the Orthopaedic Research Laboratories, Scripps Clinic Center for Orthopaedic Research and Education, La Jolla, California

The Journal of Bone and Joint Surgery, Incorporated

J Bone Joint Surg Am, 2005 Feb 01;87(2):332-338. doi: 10.2106/JBJS.C.01467

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Abstract

Background: Unicompartmental replacement can be an alternative to tibial osteotomy in younger, active patients with unicompartmental knee disease. In unicompartmental replacement, the other compartments and knee ligaments are largely untouched. Therefore, it was hypothesized that the knee kinematics after unicompartmental replacement may also be unchanged. To test this hypothesis, knee kinematics and quadriceps tension were recorded before and after replacement with a unicompartmental design and then with a tricompartmental design.

Methods: Six human cadaver knees were tested before implantation, after implantation with a bicruciate-retaining unicompartmental knee prosthesis, and after implantation with a posterior cruciate-retaining tricompartmental knee prosthesis. The unicompartmental prosthesis was initially implanted, and it was then revised to a total condylar knee replacement. The knee kinematics were measured with use of an electromagnetic tracking device while the knee was put through dynamic simulated stair-climbing under peak flexion moments of approximately 40 N-m. Quadriceps tension was also measured for all three conditions.

Results: No significant differences in tibial axial rotation were noted between the intact and unicompartmental conditions. However, tricompartmental replacement significantly affected tibial axial rotation (p = 0.001). Femoral rollback was not significantly affected by either unicompartmental or tricompartmental arthroplasty. Quadriceps tension was also similar among all three conditions.

Conclusions: In this in vitro cadaver study, the tricompartmental replacement significantly changed knee kinematics while the unicompartmental replacement preserved normal knee kinematics.

Clinical Relevance: The results of this in vitro biomechanical cadaver study suggest that the unicompartmental design has the potential to restore (or preserve) normal kinematic function better than tricompartmental implants. Restoration of normal knee function may benefit patient rehabilitation, extensor function, implant survival, and wear.

Figures in this Article

Total knee replacement is a successful procedure. The relief of pain and the restoration of function can be dramatic, and the rate of survival of the implants is long enough to satisfy most patients requiring knee replacement^1-5. However, it is often difficult to justify total knee replacement in young or middle-aged patients with unicompartmental disease. These patients are often more athletic and expect better function than do the typical patients with tricompartmental disease. The long-term outcome of high tibial osteotomy has not been consistent, with a decrease in the rate of survival to 75% at ten years and 65% at fifteen years⁶. Unicompartmental knee replacement is an attractive alternative for this patient population^6,7.

Knee kinematics have been extensively studied in normal knees, knees with ligament deficiencies, and knees that have had a total arthroplasty^8-10. There is a general consensus that total condylar (tricompartmental) knee arthroplasty substantially changes the kinematic profile of the knee^11-14. This may be attributed to several factors including the differences between the geometry of the normal articular surface of the knee and the replacement prosthesis, loss of the anterior and/or posterior cruciate ligaments, and altered neuromuscular patterns due to preexisting pathological conditions.

Theoretically, unicompartmental knee replacement offers the potential to restore knee kinematics to normal. Since only one compartment is diseased, the overall geometry of the knee is better preserved. In addition, current indications for a unicompartmental replacement necessitate the presence of an intact anterior cruciate ligament^15,16. Therefore, the ligamentous stability and soft-tissue balance of the joint can be restored more closely to normal. A recent study has suggested that knee kinematics after replacement with the Oxford unicompartmental design may be similar to that of the normal knee¹⁷. The objective of this study was, therefore, to test the hypothesis that unicondylar knee replacement does not alter normal knee kinematics during simulated stair-climbing in a cadaver model.

Materials and Methods

Materials and Methods | Results | Discussion | References

Six fresh-frozen knees from human cadavera (four male and two female donors with an age range of seventy-three to eighty-nine years at the time of death), which were free from macroscopic anatomic defects, were used in this study. Only left knees were selected, to reduce the implant inventory. The selection criteria included the absence of a severe anatomic deformity, no evidence of soft-tissue abnormality about the knee, and a bone and soft-tissue quality that was adequate to withstand the forces exerted during testing (a 1200 to 1400-N quadriceps force was required to generate up to 40 N-m of knee moment). Before implantation of the prosthesis, the knees were dissected, prepared, and mounted on the Oxford knee rig (as described below). The quadriceps tendon was loaded to 1200 N. If the extensor mechanism remained intact, the soft-tissue quality was deemed adequate for further testing. Each knee was dissected and stripped of skin, subcutaneous tissue, and muscle, leaving the knee capsule, ligaments, and quadriceps tendon intact. Electromagnetic sensors (3Space Fastrak; Polhemus, Colchester, Vermont) were fixed rigidly to the medial surface of the femoral and tibial shafts. Threaded rods were cemented into the femoral and tibial shafts for fixation to the testing rig. A nylon strap was sutured to the quadriceps tendon. The six knees were tested before implantation (intact) and then after sequential implantation with a unicompartmental knee design (Preservation; DePuy, Johnson and Johnson, Warsaw, Indiana) and a current-generation primary posterior cruciate-retaining tricompartmental knee design (PFC Sigma; DePuy, Johnson and Johnson).

The knees were mounted in a dynamic, quadriceps-driven, closed-kinetic-chain knee simulator based on the Oxford knee rig design, which has been previously described^18-21 (Figs. 1-A and 1-B). The tibial intramedullary attachment rod was fixed to the so-called ankle-joint assembly on the rig. This joint had all three rotational degrees of freedom but was constrained in translation. The femoral intramedullary attachment rod was fixed to the so-called hip-joint assembly on the rig. The hip-joint assembly was allowed freedom in flexion-extension and varus-valgus rotations, and it could slide vertically on two parallel bars. The location of the attachment of the femoral rod to the hip joint was offset laterally to account for the typical anatomic valgus (5°) at the knee (Fig. 1-B)²². The magnitude of the offset was tan(a)×d (where a was the anatomic valgus at the knee, and d was the distance between the center of the knee and the hip joint). This arrangement allows six degrees of freedom at the knee (constrained only by articular geometry and local soft tissue)¹⁹. The quadriceps tendons were attached to an electric motor through a nylon strap that applied dynamic quadriceps tension (recorded through a load-cell). A vertical load applied at the hip joint generated a peak knee-flexion moment of approximately 40 N-m, which was comparable with values measured during stair-climbing after tricompartmental replacement in in vivo studies^23,24. The quadriceps tension generated an extension moment resulting in a closed-kinetic-chain knee extension.

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Fig. 1-A: Diagram of the knee rig (based on the Oxford knee-rig design). A hip load provided the knee flexion moment, and an electric motor applied tension on the quadriceps tendon to generate the knee extension moment. The so-called hip-joint assembly was free to slide vertically and to rotate in flexion and adduction-abduction. The so-called ankle-joint assembly was free to rotate in all directions but was constrained in translation. The knee was unconstrained. This knee rig simulates a closed-kinetic-chain knee extension similar to stair-climbing or rising from a chair. (Reprinted, with permission, from: Browne C, Hermida JC, Bergula A, Colwell CW Jr., D'Lima DD. Patellofemoral forces after total knee arthroplasty: effect of extensor moment arm. Knee. doi:10.1016/j.knee.2004.05.006.)

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Fig. 1-B: A close-up view of the so-called hip-joint assembly. The vertical arrow labeled "C" points to the center of rotation of the hip-joint assembly for varus-valgus angulation. A section of the assembly could be moved in the mediolateral direction (the double-headed arrow) to create an offset between the shaft of the cadaveric femur and the center of varus-valgus rotation in the hip joint. The attachment of the femoral intramedullary fixation rod was offset laterally (arrow labeled "O") to simulate the typical anatomic valgus of the knee. The flexion axis of the hip is marked with the arrow labeled "F".

The electromagnetic tracking sensors measured three-dimensional motion during the simulated stair-climbing. Anatomic landmarks were digitized on each knee to generate embedded coordinate systems in the femur and the tibia. The midpoint of the transepicondylar line was used as the center of the femoral coordinate system, and the midpoint of the tibial plateau was used as the center of the tibial system. Tibiofemoral rotations and translations (femoral rollback) were calculated every 5° between 0° and 120° of knee flexion. Tibiofemoral rotations were calculated as follows: tibial flexion and extension were measured about the fixed femoral transverse axis (the femoral transepicondylar axis), Z; tibial internal and external rotation, about the tibial shaft axis, X; and tibial varus and valgus, about a floating axis (perpendicular to the femoral Z axis and the tibial Y axis). Femoral rollback was measured as the posterior translation of the center of the transepicondylar line of the femur relative to the fixed tibial coordinate system.

Each condition was tested three times in each knee. First, baseline intact knee kinematics (tibiofemoral rotation, tibiofemoral varus and valgus, and femoral rollback as a function of flexion) were recorded with an intact joint capsule. The capsule was then incised as is done for a routine tricompartmental replacement, the capsular incision was repaired with sutures, and the knee kinematics were rerecorded. The kinematics recorded after capsular incision and suturing were not significantly different compared with the kinematics before capsular incision for each knee. Next, the Preservation unicompartmental design was implanted and tested. This design has low tibiofemoral conformity, a cobalt-chromium-alloy femoral component, and an ultra-high molecular weight polyethylene onlay tibial component. The rationale behind the low tibiofemoral conformity is to allow for the sliding and axial rotation that normally occurs at the knee^25,26. Kinematics for the unicompartmental condition were recorded. The anterior cruciate ligament was resected, and the kinematics were recorded again. The unicompartmental components were removed, and the kinematics were recorded after implantation with the posterior cruciate-retaining tricompartmental design. The tricompartmental design had a curved-on-curved tibiofemoral articulation. The tibial component was modular with an ultra-high molecular weight polyethylene insert in a titanium-alloy tray. Standard femoral and tibial bone cuts were made according to the manufacturer's recommendations: the femoral component was placed in 3° of external rotation and 5° of anatomic valgus, and the tibial component was placed at 90° to the long axis of the tibia in the coronal plane and with no posterior slope. The femoral, tibial, and patellar implants were cemented in place. Before the unicondylar component was cemented, the back side was greased to facilitate removal. A 10-mm distal femoral cut was used for the tricompartmental replacement (measured off the lateral femoral condyle) to minimize changes in joint-line levels. The patella was resurfaced with a standard dome-shaped patellar component for the tricompartmental procedure.

Statistical Analysis

Repeated-measures multifactorial analysis of variance was used to test for differences in three dependent variables: tibiofemoral rotation, femoral rollback, and quadriceps tension. The mean differences among the conditions with respect to total tibiofemoral rotation, femoral rollback, and maximum quadriceps tension were tested first. Then, within each knee, repeated measures were tested between the four conditions (the intact knee, after unicompartmental replacement with an intact anterior cruciate ligament, after unicompartmental replacement with resected anterior cruciate ligament, and after the tricompartmental replacement) as well as at 5° increments of flexion between 0° and 120°. This would reveal any interaction effect between flexion and the dependent variables. For example, if the mean tibial rotation over the entire range of flexion was similar but the pattern of tibial rotation was different, the repeated-measures analysis of variance would detect an interaction effect between knee conditions and flexion. If the analysis of variance showed significant differences between the conditions, the following three pairwise comparisons were tested: the intact knee and the unicompartmental knee replacement, the unicompartmental knee replacement with intact anterior cruciate ligament and the unicompartmental knee replacement with resected anterior cruciate ligament, and the intact knee and the total knee replacement. Bonferroni correction was used to adjust for the three post hoc pairwise comparisons.

Results

Materials and Methods | Results | Discussion | References

In the intact knee, knee flexion was accompanied by femoral rollback. For all six knees, femoral rollback ranged between 12 mm and 20 mm and tibial rotation ranged between 15° and 30°. Qualitatively similar patterns of tibial rotation and femoral rollback were seen in the unicompartmental condition compared with the intact knee (Figs. 2 and 3). Knee kinematics after resection of the anterior cruciate ligament were not found to be significantly different from those in either the intact knee or the unicompartmental knee replacement conditions. However, a significant interaction effect between the mean tibial rotation and knee flexion was found after tricompartmental arthroplasty compared with that in the intact condition (p = 0.001) (Fig. 2). This indicated that the tibial rotation pattern over the measured range of flexion was significantly different between the intact and tricompartmental conditions.

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Fig. 2: Tibial rotation for the intact, unicompartmental, and tricompartmental conditions (the data on tibial rotation after resection of the anterior cruciate ligament after unicompartmental replacement are not shown in the interest of clarity). The values are given as the mean for six knees, with the error bars indicating the standard error of the mean. Overall, the tibial rotation curve tended to flatten out after tricompartmental replacement (p < 0.001). UKA = unicompartmental replacement, and TKA = tricompartmental replacement.

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Fig. 3: The mean rollback with knee flexion for the six knees. The error bars indicate the standard error of mean. No significant differences in femoral rollback were detected. UKA = unicompartmental replacement, and TKA = tricompartmental replacement.

When data from individual knees were analyzed, the tibial rotation pattern during knee flexion for the tricompartmental condition appeared to be offset along the Y axis from the intact condition, with loss of the original curvature. In some knees, the tibial rotation appeared to be offset toward internal rotation; in other knees, the rotation was offset toward external rotation. Since the offsets in different directions tended to cancel each other out, the mean tibial rotation after tricompartmental replacement did not appear to be substantially offset from the mean tibial rotation in the intact condition (Fig. 2). Femoral rollback (Fig. 3) and quadriceps tension (Fig. 4) did not change significantly between any of the tested conditions.

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Fig. 4: The mean quadriceps tension (in newtons) recorded during knee flexion for the six knees. The error bars indicate the standard error of mean. No significant differences in quadriceps tension were found between the conditions. UKA = unicompartmental replacement, and TKA = tricompartmental replacement.

Discussion

Materials and Methods | Results | Discussion | References

Current-generation tricompartmental total knee arthroplasty designs are very successful, with a high rate of survival of up to twenty years or more^27-30. However, return to normal function is not very common^12,31-33. Arthroplasty patients in general and those with unicompartmental disease in particular may require better function. It was hypothesized that the combination of the soft-tissue stability and balance and the minimally invasive nature of the unicompartmental knee arthroplasty could restore function to a level comparable with that of the normal knee. In the present study, a current-generation fixed-bearing unicompartmental design with low conformity was tested. Knee kinematics (rotation and rollback) during flexion after unicompartmental implantation were found to be similar to that of the intact knee, suggesting that near normal function may be expected after unicompartmental replacement, at least with the particular design used in this study.

The indications for unicompartmental knee replacement include unicompartmental disease (either medial or lateral), no evidence of substantial patellofemoral arthritis, and an intact, functioning anterior cruciate ligament^34,35. However, these recommendations have been made on the basis of trial and error and on observational cohort studies. No systematic analysis of the effect of the presence or absence of the anterior cruciate ligament has been reported. This cadaver study permitted a comparison of the importance of the anterior cruciate ligament in maintaining intact knee kinematics and quadriceps forces. Surprisingly, resecting the anterior cruciate ligament after unicompartmental replacement did not change the knee kinematics as measured in this study. This suggests that, under the conditions tested, the role of the anterior cruciate ligament in anteroposterior stability and axial rotation is not as important as the more substantial change in the surface geometry after tricompartmental arthroplasty.

Tricompartmental arthroplasty changes the entire articular geometry of the tibiofemoral and patellofemoral articulations. In addition, the alignment of the extensor mechanism is also altered (on the basis of the rotational placement of the femoral and tibial components)³⁶. Tricompartmental arthroplasty has been shown to substantially affect knee kinematics^11,33. In the present study, tibial rotational patterns were significantly affected. This was more likely due to the change in the extensor mechanism alignment or to the change in the articular surface geometry rather than to the loss of the anterior cruciate ligament. Anterior cruciate ligament resection after the unicondylar arthroplasty did not materially affect tibial rotation relative to the intact knee. Another possibility was that greater variations in tibial rotation were introduced after tricompartmental arthroplasty because the balance of soft-tissue tension around the knee may not have been restored to normal.

The precise correlation between clinical knee function and the magnitude and direction of tibial rotation has yet to be established. However, tibial rotation has been shown to be a consistent feature in deep flexion and has been associated with greater posterior translation of the lateral femoral condyle than of the medial femoral condyle³⁷. Tibial rotation generates crossing patterns of articular contact on the polyethylene insert and has been shown to affect wear^38,39. An increase in the amplitude of tibial rotation of 2° during loading contributed to significantly increased wear rates in vitro (p < 0.01)³⁸. This finding suggests that small changes in tibial rotation may be clinically relevant. The finding that individual tibial rotation was preserved after unicompartmental knee arthroplasty suggests that there were fewer changes to the anatomic and biomechanical components of the knee than there were after tricompartmental replacement, at least for the designs tested in the current study.

On the other hand, femoral rollback was not significantly affected by tricompartmental replacement. This may be because the posterior cruciate ligament was preserved in all conditions. The quadriceps forces also did not change among the different conditions tested. This was consistent with the finding of no significant change in femoral rollback. Rollback has been shown to affect the extensor moment arm and the magnitude of quadriceps forces^40,41. Experienced surgeons (C.W.C. Jr. and K.A.E.) performed the operations in relatively normal cadaver knees. It is possible that rollback in arthritic knees could be more variable after tricompartmental replacement as suggested by in vivo kinematic data^11,12,32,33.

A model of closed-kinetic-chain knee extension in the range of 0° to 120° of flexion was chosen for this study. Knee flexion angles of 50° to 60° are seen typically during stairclimbing, whereas rising from a chair may require as much as 90° to 120° of flexion. The knee rig was able to simulate these conditions in a dynamically loaded manner, and it has previously been shown to be sensitive to differences in kinematic patterns between designs^21,42,43.

Kinematic behavior tends to vary from knee to knee. Because of the wide variations among patients, in vivo comparisons are difficult to assess^{11-13,32,33,44}. In this study, the kinematics were measured for different conditions in the same knee and were treated statistically as repeated measures. In the same cadaver knee, the collateral ligaments and the posterior cruciate ligament were left intact and the quadriceps alignment and the bone lengths were maintained, thus removing potential variations due to differences in these parameters^21,43,45. In addition, differences in neuromuscular activation patterns that may be seen in vivo were also reduced, since the knee rig generated a similar extension loading profile for each condition.

This study only tested cadaver knees in controlled, closed-kinetic-chain knee extension. It is possible that the results may be different in other activities. In addition, cadaver knees without macroscopic anatomic abnormalities were selected for testing. These knees may not reflect the pathologic soft-tissue changes seen in a knee with typical tricompartmental disease. However, the lack of soft-tissue abnormalities reduced the surgical variation that may have been introduced because of differences in soft-tissue balancing. Also, the knees of patients in whom unicompartmental replacement is indicated may have less soft-tissue abnormality than those of candidates for total knee replacement.

Several apparent advances have been made in unicompartmental knee component design (low tibiofemoral articular conformity and onlay tibial prosthetic design), instrumentation (drilling and cutting guides and milling instruments), and materials (better quality of ultra-high molecular weight polyethylene resin and control of sterilization techniques). The use of unicompartmental replacement in younger, more active patients with localized knee disease is also increasing. There is some evidence that suggests that unicompartmental disease may not spread to the other compartments after a unicompartmental replacement (at least in one mobile-bearing design)⁴⁶. The encouraging results of this in vitro biomechanical cadaver study suggest that the unicompartmental design may offer the potential to restore (or preserve) normal kinematic function better than the tricompartmental replacement. The restoration of normal knee function may benefit patient rehabilitation, extensor function, implant survival, and wear. ?

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D'Lima DD, Poole C, Chadha H, Hermida JC, Mahar A, Colwell CW Jr. Quadriceps moment arm and quadriceps forces after total knee arthroplasty. Clin Orthop.2001;392: 213-20.392213 2001 [PubMed][CrossRef]

Weale AE, Murray DW, Crawford R, Psychoyios V, Bonomo A, Howell G, O'Connor J, Goodfellow JW. Does arthritis progress in the retained compartments after `Oxford' medial unicompartmental arthroplasty? A clinical and radiological study with a minimum ten-year follow-up. J Bone Joint Surg Br.1999;81: 783-9.81783 1999 [PubMed][CrossRef]

Topics

cadaver ; femur ; knee joint ; tibia ; arthroplasty ; knee replacement arthroplasty ; kinematics ; quadriceps ; knee flexion

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Darryl D. D'Lima, M.D.

Posted on March 16, 2006

Dr. D'Lima et al respond to Dr. John

Scripps Clinic Center for Orthopaedic Research and Education, La Jolla, CA 92037

Dr. John may be right in his statement that translation of the mid- point of the transepicondylar axis would not by itself capture the differential rollback between the lateral and medial femoral condyles. However, differential rollback would be recorded as axial tibial rotation. In the knee biomechanics literature, kinematics are expressed differently depending on the type of motion analysis used. Typically, fluoroscopic studies assume that the lowest point on the femoral condyle represents the instantaneous tibiofemoral contact and report translation of this point as rollback. Motion analysis studies that use skin or bone mounted tracker systems typically use anatomic landmarks to define a coordinate system in the femur and report translation of the center of this coordinate system. We chose the latter for direct comparison of kinematics between our own previous studies and the large available body of knee kinematic literature.

If differential rollback changed after unicompartmental arthroplasty this would be reflected in a change in translation of the mid- transepicondylar point and/or a change in axial tibial rotation. Defining rollback in either fashion would not change our conclusion. Dr. John is entitled to his opinion regarding the cause of relatively higher lateral unicompartmental replacement failure. However, if he wishes to support his speculation we would like to see evidence that increased rolling produces more wear that sliding. The use of term â€œtranslationâ€ does not differentiate between rollback and sliding. Greater posterior translation of the lateral femoral condyle relative to the medial indicates greater rolling in the lateral compartment and greater sliding in the medial compartment. When we mention the advantages of normal kinematics, we were comparing it with the abnormal kinematics reported after total knee replacement which involve forward sliding of the femur with flexion (â€œreverse rollbackâ€)(1).

Reference:

1. Dennis, D. A., Komistek, R. D., Colwell Jr, C. W., Ranawat, C. S., Scott, R. D., Thornhill, T. S., and Lapp, M. A.: In vivo anteroposterior femorotibial translation of total knee arthroplasty: a multicenter analysis. Clin Orthop Relat Res.47-57, 1998.

Darryl D. D'Lima, M.D.

Posted on March 16, 2006

Dr.D'Lima et al respond to Drs. Joshi and Nagare

Scripps Clinic Center for Orthopaedic Research and Education, LA Jolla, CA 92037

In our publication we did not recommend performing unicompartmental replacement in ACL deficient knees. Our article states that we only tested knees without obvious defects. In these knees transection of the ACL would not simulate the entire clinical spectrum seen with traumatic or pathological ACL rupture. An ACL deficient knee with unicompartmental arthritis may have other associated abnormalities that could affect survivorship.

One of our goals was to determine whether the change in knee kinematics after total knee arthroplasty could be attributed to the missing biomechanical support provided by the ACL. Since transecting the ACL did not change the kinematics after unicompartmental replacement, we concluded that resecting the ACL was not likely to be responsible for abnormal kinematics observed in total knee replacement. We can only repeat what we stated in our article: â€œThis study only tested cadaver knees in controlled, closed-kinetic-chain knee extension. It is possible that the results may be different in other activities.â€

We strongly caution against unicompartmental replacement in ACL deficient knees until more definitive evidence is available.

Joby John

Posted on November 17, 2005

Can Normal Knee Kinematics Be Restored with Unicompartmental Knee Replacement

Royal Shrewsbury Hospital, UK

To The Editor:

I read with great interest the article by Patil, et al. I agree with the authors that preservation of anatomy in unicompartmental knees would produce a near normal kinetic and kinematic profile. However, the definition of roll-back as the translation of the mid point of the transepicondylar axis, would seem questionable. It is known that the lateral condyle translates substantially more than the medial condyle. Femoral condylar roll-back is said to occur only laterally.(1) Measuring roll-back with a point on an axis closer to the medial condyle would not be reflective of the true picture of roll-back, as small alterations in translation of tibiofemoral contact point on the lateral compartment may not be detected.

This may be the reason why it seemed that ACL resection did not affect the knee kinematics in this study. Results from unicompartmental arthroplasty series have shown inferior results in ACL deficient knees.(2) This phenomenon is unlikely to happen if the kinematic profile of the knee was unaltered. I would also be skeptical about the conclusion that restoring normal kinematics of the knee would decrease the wear. Lateral unicompartmental knee replacements have not done as well as their medial counterparts, partly because of the decreased tibial slope and also increased tibiofemoral translation. It might actually be a case of the normal kinematics being partly responsible for the decreased survival of the lateral unicompartmental knee replacement.

References:

1. Pinskerova V., et al. Does femur roll-back with flexion? JBJS Br.2004 Aug;86-B(6):925-931.

2. Goodfellow JW. The Oxford Knee for unicompartmental osteoarthritis. JBJS Br.1998 Nov;70(5):692-701.

Avinash P Joshi

Posted on March 17, 2005

Can normal Knee kinematics be restored with unicompartmental Knee replacement ?

Gloucester Royal Hospital, Great Western road, Gloucester, GL1 3NN, UK

To the Editor:

We read with great interest the recent article by Patil, et al. We agree with the authors that when compared to performing TKA for triccompartmental disease, there is 'lack of soft tissue abnormalities further reducing surgical variations' in a unicompartmental knee. However, we would like to stress that 'balancing of the soft tissue tension' in a Knee undergoing unicompartmental replacement is equally important and at times very difficult. In fact a minor change in the thickness of the polythelene spacer affects the tibio- femoral angle and hence also the tension around the medial collateral ligament in a unicompartmental Knee(1).

We would also disagree with their conclusions that 'the role of ACL in antero-posterior stability and axial rotation is not important'. This is when we know the results of a unicompartmental Knee to have been inferior and to have been failed in a ACL deficient/lax Knee(2).

The paper also states that resecting the ACL in a unicompartmental Knee arthroplasty did not change the kinematics of the Knee. Does one conclude from the findings that a unicompartmental Knee performed in a ACL deficient/lax Knee could give the same results and equal chance of success/failure as in a ACL intact Knee ?

References:

1) Hopgood.P et al. The effect of tibial implant size on post- operative alignment following medial unicompartmental knee replacement. Knee. 2004 Oct;11(5):385-8.

2) Goodfellow JW. The Oxford Knee for unicompartmental osteoarthritis. JBJS Br.1998 Nov;70(5):692-701.

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Shantanu Patil, Clifford hW. ColwellJr., Kace A. Ezzet, Darryl D. D'Lima; Can Normal Knee Kinematics Be Restored with Unicompartmental Knee Replacement?. The Journal of Bone & Joint Surgery. 2005 Feb;87(2):332-338.

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