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ORIGINAL ARTICLE

Analysis Of Alignment Of Uni Compartmental Knee Replacements Implanted Over A Period Of 15 Years

* J John, PC May

*Princess Royal Hospital, Telford, United Kingdom

Address for Correspondence

J John,
Princess Royal Hospital, Telford, United Kingdom

Abstract

Background: In uni compartmental knee replacement, the other compartments and knee ligaments are largely untouched. Therefore, the knee kinematics after uni compartmental replacement may also be unchanged. The results of these uni compartmental knee replacements would depend on maintaining alignment, so that the unaffected compartment is not overloaded. The maintenance of the mechanical axis closer to the centre of the knee also has a bearing on the rate of polyethylene insert wear.  
Methods: Uni compartmental knee replacements implanted over a period of 15 years by one surgeon was reviewed and analysed for the alignment using the mechanical axis. The tibial plateau was assigned zones 0, 1, 2, C, 3, 4, 5. 0 being in extreme varus and 5 being in extreme valgus. The appropriate alignment was defined as varus or central for medial unicompartmental knee replacements and valgus or central for lateral uni compartmental knee replacements. 
Results: 71% of the knees analysed had the ideal alignment. 23% had an acceptable alignment. 6% had unacceptable altered alignment. 30% of the knees with unacceptable altered alignment had to undergo conversion to TKR.
Conclusions: Maintaining alignment in uni compartmental knee replacements is of paramount importance, to prevent the accelerated degeneration of the unaffected compartment. Significant alteration of alignment results in poor results.

J.Orthopaedics 2006;3(2)e3

Introduction:

Replacement arthroplasty of the knee 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, 9). It is difficult to justify total knee replacement in patients with uni compartmental disease. These patients are treated with upper tibial osteotomy or uni compartmental knee replacement. 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 years (13). Uni compartmental knee replacement is an attractive alternative for this patient population (13).

The general consensus is that total condylar (tricompartmental) knee arthroplasty substantially changes the kinematic profile of the knee (12). 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. Improved kinematic profile with good long term survivorship at 10 yrs makes uni compartmental knee replacements in uni compartmental knee arthritis a very attractive proposition (8).  

Theoretically, uni compartmental knee replacement offers the potential to restore knee kinematics to normal. The overall geometry of the knee is better preserved as only one compartment is replaced. In addition, current indications for a uni compartmental replacement necessitate the presence of an intact anterior cruciate ligament (5, 8). Knee kinematics after replacement with the Oxford uni compartmental design may be similar to that of the normal knee (10). Therefore, the ligamentous stability and soft-tissue balance of the joint can be restored more closely to normal and would influence the kinematic and kinetic profile of the knee. The final alignment of the knee is a static reflection of these factors. The objective of this study was to analyse the mechanical axis in patients who have had uni compartmental knee replacements from1989 to 2004.

Material and Methods :

Between 1989 and 2004, 92 uni compartmental knee replacements were performed by the same surgeon (PCM) using the same design of prosthesis (Miller- Galante). 84 knees were medial uni compartmental knee replacements and 8 knees were lateral uni compartmental knee replacements. Of these 55 knees were available for follow-up, 27 were dead and 4 lost to follow-up. These 55 knees were subjected to long leg views to assess the mechanical axis and the zone through which the mechanical axis passed, as described by Kennedy and White. The mechanical axis is defined as the line passing from the centre of the hip to the centre of the ankle. The zone through which this line passes as described by cartier and villiers is further assessed.

The above figures illustrate the mechanical axis (on the right) and the zone of passing of the mechanical axis on the left.

Results :

55 knees were assessed. All except one medial uni compartmental knees were still in varus. The one medial uni compartmental knee replacement which had an altered alignment was caused secondary to a total hip replacement. 36 knees( 66 %) had the mechanical axis pass through zone C or 2 .13 knees( 24 %) had the mechanical axis pass through zones 1 and 0. 71% of the knees had the ideal post- operative alignment we had aimed for. The alignment  over a long period of time, especially considering that poly ethylene wear and component subsidence would have resulted in alteration of alignment.

Discussion :

Indications for uni compartmental knee replacement include uni compartmental disease (either medial or lateral), no evidence of substantial patello femoral arthritis, and an intact, functioning anterior cruciate ligament (7). In our series the radiographic changes in the patello femoral joint were considered relevant only if they produced clinical symptoms. Several authors have emphasized the need for under correction of the deformity during uni compartmental knee replacements in order to minimize the degeneration in the unaffected compartment (4, 6). Alignment in excessive varus in medial uni compartmental knee replacements also result in accelerated wear and tibial component subsidence (6). Most authors believe that the ideal alignment is with the mechanical axis passing through Zone C or Zone 2 in case of medial uni compartmental replacements and Zone C or Zone 3 in case of lateral uni compartmental replacements (2, 6). Hence the long term success of the surgery would be dependent on the reproduction of the alignment. Collateral ligament laxity also plays an important part in the maintenance of post operative alignment, and needs to be assessed clinically and if necessary with stress radiographs (2, 6). Although a release of the medial collateral ligament has been proposed (12), it was not performed in our series, as we were concerned about leaving the replaced compartment lax, especially in cases of mobile bearing uni compartmental knees as this may result in the dislocation of the mobile meniscus (5). Moreover in our view if the knee required significant soft tissue release a uni compartment knee replacement may be an inappropriate procedure. Lateral uni compartmental arthroplasty is much less common than medial uni compartmental knee replacement (11). The slope of the lateral tibial plateau is lesser than the medial tibial plateau and hence the wear pattern is posterior as opposed to antero medial in case of medial compartment. Lateral compartment replacements should hence be placed with tibial resection made with less posterior slope (11). Care should be taken to prevent alteration of alignment while performing lateral uni compartmental knee replacements. Majority of the revisions in our experience were in lateral compartment disease, due to accelerated degeneration in the uninvolved compartment, secondary to alterations in the alignment.

Conclusion:

1) The maintenance of alignment in uni compartmental knee replacement is of paramount importance in preventing progression of osteoarthritis in the normal compartment and wear in the prosthesis.
2) Appropriate alignment can be reproduced in majority (99%) of the medial uni compartmental knees with adequate care before and during surgery.
3) Lateral uni compartmental knee replacement require more care partly because they are rarely performed and in our experience more likely to result in an altered alignment.
4) Only 30% of the knees with unacceptable altered alignment required conversion to TKR.

Reference :

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  6. Kennedy RK, White RP. Unicompartmental  arthroplasty of the knee : Post-operative alignment and its influence on overall results. Clin Orthop 1987; 221: 278-85.

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  8. Murray DW, Goodfellow JW, O'Connor JJ. The Oxford medial unicompartmental arthroplasty: a ten-year survival study. J Bone Joint Surg Br.1998; 80B:983 9.

  9. Nafei A, Kristensen O, Knudsen HM, Hvid I, Jensen J. Survivorship analysis of cemented total condylar knee arthroplasty. A long-term follow-up report on 348 cases. J Arthroplasty. 1996; 11: 7-10.

  10. Robinson BJ, Rees JL, Price AJ, Beard DJ, Murray DM. A kinematic study of lateral unicompartmental arthroplasty. Knee. 2002;9: 237 -40.

  11. Scott RD. Lateral Unicompartmental Replacement: A road less traveled. Orthopaedics. 2005; 28(9): 983-4.
    12) Stiehl JB, Komistek RD, Dennis DA, Paxson RD, Hoff WA. Fluoroscopic analysis of kinematics after posterior-cruciate-retaining knee arthroplasty. J Bone Joint Surg Br. 1995;77B: 884 9.

  12. Weale AE, Newman JH. Unicompartmental arthroplasty and high tibial osteotomy for osteoarthrosis of the knee. A comparative study with a 12- to 17-year follow-up period. Clin Orthop. 1994;302: 134 -7.

This is a peer reviewed paper 

Please cite as : J John: Analysis Of Alignment Of Uni Compartmental Knee Replacements Implanted Over A Period Of 15 Years

J.Orthopaedics 2006;3(2)e3

URL: http://www.jortho.org/2006/3/2/e3

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