UNCOMPARTMENTAL KNEE REPLACEMENT

Abstract

There are now a number of controlled prospective trials comparing the advantages of unicompartmental arthroplasty versus total knee replacement (Rougraff 1991, Lawrencin 1991, Newman 1998, and Price 2000). These studies all favour unicompartmental arthroplasty over total knee replacement in terms of the following. The kinematics of uncompartmental arthroplasty are better and more normal for the surgery retains both cruciate ligaments with proprioceptive input. The range of motion tends to be greater in the unicompartmental group and the function better. This is especially true of demanding activities such as ascending and descending stairs, and has been shown using gait study analysis (O’Connor 1986).

The pain relief is as good, or better, with unicompartmental arthroplasty in these studies when compared to total knee replacement, and in particular there is “a better feel” with unicompartmental arthroplasty. The complications with the smaller procedure tend to be less frequent and severe and the recovery more rapid, with a potential benefit allowing for a lower cost.

There are however certain disadvantages encountered in using unicompartmental arthroplasty when compared to total knee replacement. In general there is a higher revision rate with the unicompartmental arthroplasty and this is particularly borne out in the Swedish Knee Arthroplasty Register. Using the strict criteria of Insal/Stern they suggest an incidence of 1:20 patients suitable for unicompartmental arthroplasty, and with such small numbers it is hardly surprising that there is a higher complication rate.

The advantages of unicompartmental arthroplasty in the young remains controversial. In particular there are few comparative studies (Broughton and Newman 1988).

In a small study from Oxford we have found that the pain relief and function in the unicompartmental arthroplasty group were substantially better with an age match comparison group using a patient based question score (The Oxford Knee Score 0–48). We sent the Oxford questionnaire to the HTO patients of the main proponent of osteotomy surgery in the UK, who has devoted a lifetime to perfecting the art of osteotomy surgery. These patients represent the “best case scenario” and his patients at five year follow up scored 27/48 on the Oxford Knee Score. Age match group of young uni-compartmental arthroplasty patients scored 38. Comparison of total knee replacement group would score 35, and it is of interest to note that those patients revised from a failed high tibial osteotomy to a total knee replacement raised their scores from 27 to 33. There are of course disadvantages comparing unicompartmental arthroplasty and high tibial osteotomy in the young. The main disadvantages that artificial material is implanted and there is the potential for infection. What remains debatable and controversial is the outcome of procedures when converting them to a total knee replacement. In terms of 10 year survivorship most of the published literature suggests that with high tibial osteotomy there is a two-thirds survivorship, one-third being converted to a total knee replacement by 10 years (range 51% to 80%) (Naudie 1999, Coventry 1993, Rudan 1991). The similar 10 year survivorship of unicompartmental arthroplasty in patients under 65 years is in the order of 80%. This is borne out in the Swedish Knee Registry.

In general the problem with unicompartmental arthroplasty seems to centre around a higher revision rate, and faced with this problem there are a number of solutions. One can accept this and abandon the procedure, which has happened until recently in the United States. One can suggest that a unicompartmental arthroplasty is used as a pre-knee replacement, which has been forwarded by Repicci in the States. Alternatively one can try to minimise the failure rate by employing an implant with very good wear characteristics, one can concentrate on appropriate indications and one can define an accurate reproducible technique. One can seek to achieve a survival rate that is similar to that of the best total knee replacement.

The Oxford unicompartmental knee replacement was deigned first by Goodfellow and O’Connor 25 years ago. It employs a spherical femoral component articulating on a flat tibial component. There is a fully mobile bearing, which is unconstrained. This bearing is fully congruent in all positions, which minimises wear.

In two published retrieval studies (Argenson and Psychoyios) 10 year wear rate was 0.03mm per year. In those cases with no impingement the rate was 0.01mm per year. There was no correlation with thickness and we now feel comfortable advising a 3.5mm bearing for long term survivorship.

The indications for unicompartmental arthroplasty in essence centre around medial compartment osteoarthritis with a functionally intact ACL. Some superficial damage to the ligament is acceptable, but in essence the structure needs to be intact to be functioning. There needs to be a correctable varus deformity with full thickness lateral compartment articular cartilage and this is best demonstrated on stress x-ray. A fixed flexion deformity of less than 15° is usual and employing the above indications we find that a unicompartmental arthroplasty is suitable for 1:4 knees presenting with osteoarthritis.

We do not feel that the state of the patellofemoral joint is a contraindication to unicompartmental arthroplasty. We have significant evidence to corroborate this statement. In Mr Goodfellow’s published series in 1998 the state of the patellofemoral joint and the clinical results did not correlate. The study by Weale (1999) there was no progression of patellofemoral osteoarthritis over 10 years. On the Swedish Registry there have been no cited revisions for progression of patellofemoral arthritis.

The age and the activity of the patient does not seem to be an obvious contraindication. In particular in the old and unfit using the minimal invasive approach there is a low morbidity, with all its attendant advantages. In the young patient (less than 50 years), the 10 year survivorship is 92% in two published series (Murray et al 1998 and Price 2000).

The published 10 year results of the designers patient (Murray et al 1998) details the follow up of 144 unicompartmental arthroplasties with a 10 year survivorship. At 10 years there were 34 knees at risk giving a 98% 10 year survivorship 95%, confidence levels 93–100%. There was one case lost to follow up giving a worse case scenario of 97%.

Of much more relevance concerns an independent series from Sweden (Svard et al 2001). These series is of 420 Oxford unicompartmental arthroplasties from a single centre performed by four surgeons. None lost to follow up. A 10+ year follow up involved 122 Oxford unis reviewed, with 92% good or excellent HSS scores.

The 15 year survivorship was 94% with confidence levels 86 to 100%, there were none lost to follow up so the 15 year worse case scenario was 94%. This is better than fixed bearing unis and as good as the best total.

The Swedish Knee Arthoplasty Register however gave a different picture, and was published in 1995 (Lewald et al), reported poor early results with no learning curve and advised that the difficult implant should not be used. We in fact have gained data from 944 rather than 699 from the register. It concluded that at these centres they had very reasonable results, but one or two centres had catastrophically poor results, in the order of 30% failure. We can only conclude that these poor results were due to inappropriate indications or technique. More recently in January 2001 Robertson et al have published an update of the Swedish Knee Arthroplasty Register citing good to excellent results in those centres performing more than 23 Oxford unicompartmental arthroplasties a year. Good results were possible, but there is a definite learning curve.

The phase 3 tradition of the Oxford was introduced in 1998. The aim of this introduction was to make the operation simpler and more consistent. We have consistently employed a minimally invasive approach, but we have sought to keep the advantages of phase 2 Oxford unicompartmental arthroplasty. In effect there has been minor modifications to the instruments with an increased range of sizes.

Our early phase 3 results, published in 1999 (Price et al) have compared the early recovery. This is the time taken to functional recovery, by which time the patient is ready for discharge. We compared the first 40 minimals with the last 20 opens and used 40 knee replacements taken as controls performed at the same time. We have shown that the minimally invasive unis recover three times faster than the totals (p< 0.001) and twice as fast as the open unis (p< 0.001).

Finally our one year follow up of the first 58 phase 3 Oxford unicompartmental arthroplasties reveal increase in the mean flexion from pre-operative 123° to postoperative 135°. A high proportion of the patients gained at least 130° of flexion and 50% were 140+. A mean AKS score rose from a pre of 37 to one year of 98. The AKS function score raised from a pre 53 to one year 94, with a very high proportion of patients scoring 95+ score out of 100 on the AKS.

In summary unicompartmental arthroplasties offer many potential advantages over TKR in terms of:

• - Recovery, function.

• - The best long term results of uni (Oxford) are now as good as best TKR.

• - Unis in general are technically demanding and there is a definite learning curve.