Unicompartmental Knee Arthroplasty
N.P. Kort, MD, PhD
N.P. Kort, R. Deutman, J.J.A.M. van Raay and J.R. van Horn
Martini Hospital, Postbus 30033, 9700 RM Groningen, The Netherlands.
Prof. J.R. van Horn (orthopaedic surgeon), University Medical Center Groningen, Hanzeplein 1, 9713 GZ Groningen, the Netherlands.
The Oxford unicompartmental knee replacement has recently gained popularity for the treatment of medial compartment gonarthrosis thanks to the excellent long-term results of replacement with a mobile polyethylene bearing. Moreover, it is now possible to implant this prosthesis via a minimally invasive approach. In this study we analysed the results and complications of such procedures. From December 1998 to July 2001, 65 patients with this condition were operated using the minimally invasive approach. A revision was performed in four cases. In one patient a non-dislocated tibial plateau fracture was treated conservatively. One patient died 14 months postoperatively due to cardiac arrest unrelated to the operation. The results of the remaining 60 patients after a mean follow-up of 12 months (6-36 months) were good to excellent.
When conservative measures offer no permanent solution for pain caused by anteromedial arthrosis of the knee, a corrective tibial head osteotomy, a unicompartmental knee prosthesis or a total knee prosthesis are considered. Placing a unicompartmental knee prosthesis seems logical, as the lateral joint compartment is not damaged.
The first unicompartmental knee prostheses were placed in the 1950s and 1960s by respectively McIntosh and McKeefer (1,2). The medial tibial plateau was replaced by an acrylic or steel component. In the early 1970s, Gunston and Marmor introduced one of the first cemented metal (femoral component) on polyethylene (tibial component) unicompartmental knee prostheses (3,4). In 1988, Marmor published disappointing results after 10-13 years (5,6). In about 20% of the cases there was wear of the polyethylene or a symptomatic arthrosis of the contralateral compartment. The Swedish joint replacement registry also published unsatisfactory results, mainly because unicompartmental knee prostheses were also placed for chronic inflammatory conditions (7). Other authors also indicate revision operations in 20-28% of the cases (8,9). For this reason, unicompartmental knee prosthesis became less popular in the 1980s.
New prosthetic designs have been introduced by now. Although the results of the unicompartmental knee prostheses with a fixed polyethylene tibial plateau attached to a metal bearing were moderate, the results of the Oxford unicompartmental knee prosthesis with a mobile polyethylene bearing are very promising (figures 1 and 2). A precondition is that the prosthesis be used for the treatment of anteromedial compartment gonarthrosis with an intact anterior cruciate ligament (10-18). Thanks to an improved instrumentarium, for a few years now it has been possible to place the unicompartmental knee prosthesis via a small incision, which compared to conventional arthrotomy shows a more limited morbidity, an early functional recovery up to two times faster, and almost half the blood loss (19,20). By early functional recovery we mean the time that is needed to climb stairs and bend the knee joint at least 70 degrees (20).
Precisely thanks to the possibility of a minimally invasive approach, in the Netherlands too there has been renewed interest in the Oxford unicompartmental knee prosthesis in recent years (figure 3). For some, corrective tibial head osteotomy is even losing its position as the gold standard (21-25). A comparative study by Broughton into the results of corrective tibial head osteotomy as against those of a unicompartmental knee prosthesis show respectively 43% and 76% good results (18). Late degeneration in the lateral compartment was not seen after placement of a unicompartmental knee prosthesis. Weale and Newman report better function and longer survival of the unicompartmental knee prosthesis compared to tibial head osteotomy (21). More recent studies also show a quicker full carriability, easier rehabilitation, fewer perioperative complications and better long-term results after a unicompartmental knee prosthesis compared to tibial head osteotomy.
This article describes the first outcomes of a prospective study into the results of the Oxford unicompartmental knee prosthesis of the first 65 patients who were operated via the minimally invasive approach.
Sixty-five patients were operated in the period between December 1998 and July 2001. The patients were included on the basis of an international study protocol, the ‘Oxford Unicompartmental Knee Prosthesis Phase 3 study’. It included 43 women and 22 men with an average age of respectively 67 (53-83) and 65 (52-75) years. Average body height was 169 (149-194) cm and average body weight 85 (53-110) kg. Forty-five patients had their right knee operated and 20 patients their left knee.
All patients had anteromedial gonarthrosis with an intact lateral knee compartment, an intact anterior cruciate ligament and a varus axis deviation that could be easily corrected passively (11,26-28). The presence of patellofemoral arthrosis was not seen as a contraindication (26). Medical histories of all patients were recorded on a special questionnaire form. Use was also made of the ‘Knee Society Score’ (KSS). The KSS is subclassified into an anamnestic (ADL) score and a clinical score. The clinical score is formed by a knee score after physical examination and a pain score. A maximum of 100 points can be attained. Results are classified as follows with the KSS: 85-100 points is excellent, 70-84 points is good, 60-69 moderate and <60 poor.
The WOMAC score indicated the pain, stiffness and functional scores. General well-being was indicated in the SF-36 questionnaire. Both questionnaires were filled in by the patients themselves. The Visual Analogue Score (VAS score) was used for the patients to indicate degree of pain with a figure between 0 and 10 points (0: no pain to 10: extreme pain). Average knee flexion amounted to 124 (100-145) degrees preoperatively.
The operation was done with a hanging lower leg such that the knee could bend at least 120 degrees. After cementing both metal components, a thickness for the mobile bearing was chosen so that the medial ligamental structures could be tightened again with a simultaneous recovery of the physiological (anatomical) leg axis.
Postoperative mobilisation of the patient was loaded on the first day under the guidance of a physiotherapist, using two elbow crutches. Active bending and stretching exercises were also started on the first postoperative day. Patients were checked postoperatively on an outpatient basis after 3, 6 and 12 months, and then yearly. At every check-up, standardised loaded X-rays were made in two directions, while the results of the intervention were scored with the above-mentioned score lists.
Due to malposition, in one patient a revision operation was necessary in which only the tibial component was exchanged. No other perioperative or direct postoperative complications occurred. A second patient fell within three months. When he came for the check-up he had no problems and a normally loadable leg. X-rays showed a nondisplaced fracture of the tibial plateau (Figure 4). Treatment was not necessary. He remained symptom-free until his death due to cardiac problems 14 months after the knee operation. On a third patient, refixation of a loosened femoral component was necessary after six months, and on a fourth patient conversion into a total knee prosthesis was needed within one year due to loosening of both components. On a fifth patient the unicompartmental knee prosthesis was converted into a total knee prosthesis with luxation of the mobile bearing as a result of malposition of the tibial component (Figure 5).
Operation time was 88 (60-190) minutes. No postoperative blood transfusions were necessary. Hospital stay was on average 6 (4-9) days. The other 60 patients had an average follow-up of 12 (6-36) months. Among them, two patients had a 3-year follow-up
The WOMAC score showed a postoperative positive tendency for pain, stiffness and functional scores (Figure 6). The KSS showed a positive tendency for the ADL and the knee scores (Figure 7). The average preoperative KSS score amounted to 49 points (poor), while 3, 6 and 12 months postoperatively there were average scores of respectively 76, 78 and 82 points (good). At the 2- and 3-year check-up, excellent results were seen with an average of 88 points. The results of the SF-36 questionnaire showed that especially the mental condition of the patients and a number of social factors improved (Figure 8). In the VAS score a clear pain reduction was measurable with a drop of the average VAS score from 9 to 2 points.
Physical examination showed the average scar size to be 7 (5-11.5) cm long. In 75% of the cases, there was a stable clinically neutral leg axis and a remarkably good walking pattern. The average knee flexion was 120 (90-150) degrees; in one patient, a knee flexion of 90 degrees was observed at check-up after three months. Postoperatively, none of the patients showed a reduction of knee stretching compared with the preoperative situation. X-rays did not show more than 2 mm radiolucency of the femoral or tibial components in any of the 60 patients.
Encouraged by the good long-term results of the Oxford unicompartmental knee prosthesis with a mobile bearing, in recent years there has been a renewed interest in the unicompartmental knee prosthesis. The cumulative survival rate varies namely from 95% to 99.1% after 10 years, and the percentage of revisions from 0.83% to 10% (10-18). In addition to better results, popularity has increased largely thanks to the possibility of placing the prosthesis via a small incision; in particular, improvement of the instrumentarium has made this possible. In our experience too, placing the prosthesis in this way goes very well. Based on current experience, the added value of the minimally invasive operative technique consists of little morbidity, quick early functional recovery and a short hospital stay. The study of Price shows that, with this approach, recovery is twice faster than with implantation of a unicompartmental knee prosthesis using a conventional approach, and three times faster than with a total knee prosthesis (29). The apparent advantages of the minimally invasive approach made us decide at that time to place the Oxford unicompartmental knee prosthesis.
The results of our 60 patients are good to excellent up to now. This is manifest in the improved results in the various scoring lists used. Wheale et al. describe an increase of the average preoperative KSS score from 39 to 85 points at the 1- and 2-year follow-ups. This is comparable with the average KSS score of our study – an increase from 39 to 88 points. Keys described a hospital stay of 5.2 days with the minimally invasive approach, compared to our 6 days, while a conventional approach involves an average hospital stay of 8.3 days (19). The improved results in the WOMAC and SF-36 score list cannot be compared to other studies because they were not used in such studies. The 6.2% percentage of revisions in our study is relatively high, at a variance of 0.83% to 10% in other studies with a 9- to 10-year follow-up (12,18). The most probable explanation is the learningcurve for the operative technique and for establishing an indication.
In general, a strict indication is determinant to the chances of a good treatment result. The Oxford unicompartmental knee prosthesis is above all suitable for patients with anteromedial gonarthrosis with an intact anterior cruciate ligament. Results of the unicompartmental knee prosthesis in other joint conditions are not as good (11-13,16,18). One patient needed revision of the prosthesis due to loosening, showing a picture of chronic erosive arthritis, possibly rheumatoid arthritis. The problems of the other four patients were of an operative-technical nature. The tibial plateau fracture that arose after an adequate trauma was possibly related to the vertical sawing action just next to the intercondylar eminence (Figure 4). One patient needed revision within 1 week due to an unacceptable position of the tibial component, and one patient needed a relatively quick revision due to an aseptic loosening of the femoral component. So far, in 3½years we have seen one luxation of the mobile bearing (Figure 5). Upon revision of this patient a malrotation of the tibial component was identified as cause of the luxation.
To summarise, our first experiences with the Oxford unicompartmental knee prosthesis using a minimally invasive approach have been good. The complications described can be explained by a learningcurve related to the operative technique and the indication. On the basis of our first experiences with the Oxford unicompartmental knee prosthesis via a minimally invasive approach, we believe that the current results justify a further adjustment of this treatment method.
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The components of the Oxford Phase 3
unicompartmental knee prosthesis
Postoperative X-rays of a unicompartmental knee prosthesis with a mobile bearing
(A) anteroposterior view (B) lateral view
The minmally invasive approach
A nondisplaced fracture of the tibial plateau
Luxation of the mobile bearing
The Knee Society Score
The SF-36 score