The full thickness of the cartilage is taken to be evidence that it is adequate to sustain load even if its surface is fibrillated and has superficial erosions.

Fibrillation and chondromalacia are almost always present in parts of the lateral (and patellofemoral) compartments of knees with anteromedial OA, probably because of the chronic synovitis present throughout the joint cavity and the abnormal loading regime experienced by the cartilage as a result of the varus deformity. It is because surface changes have proved largely irrelevant in predicting long-term outcome that arthroscopy does not appear among the necessary preoperative investigations listed above. Indeed it can overestimate the surface damage and confuse the issue.

However, thinning of the articular cartilage in the lateral compartment is taken to be a sign of its impending failure to support load and is a contraindication to UKA. Therefore, accurate measurement of cartilage thickness is important. We have tried ultrasound imaging and MRI, but have found these methods less reliable than the valgus-stressed plain radiograph (which has the added advantage of showing, at the same time, whether the varus is correctable).

The justification for the advice given above rests mainly on several published 10-year survival studies for OUKA performed using these criteria. In one study, there was no radiological evidence of deterioration during that time. Table 4.1 compares the appearance of the lateral compartment on radiographs taken immediately postoperatively with its appearance 10+ years later (mean interval 11.4 years) (Weale et al., 1999). The films were all taken under fluoroscopic control and therefore were strictly comparable. The intra-observer error was small (K = 0.64) for the Ahlback classification and moderate (K = 0.44) for the Altman grading (Ahlback, 1968; Altman et al., 1987). One lateral compartment in 23 knees examined showed definite progression of arthritis, but only on the Altman classification. Statistical analysis of the scores revealed no significant deterioration with time.

Table 4.1 Radiological evaluation of the lateral tibiofemoral compartment 10+ years after OUKA compared with its status at one year. Each pair or radiographs were assessed twice

Condition Ahlback classification Altman classification

(The table is reproduced with permission and copyright © of the British Editorial Society of Bone and Joint Surgery [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? J Bone Joint Surg [Br] 1999; 81-B: 783–9].)

Nevertheless, failure of the lateral compartment has been a cause for revision of OUKA (see Chapter 10). The question as to whether the failure rate from this cause could be diminished by employing different, or more stringent, preoperative criteria than those advised above will only be answered by further prospective long-term studies. This matter is discussed in Chapter 10 where it is concluded that many lateral compartment failures result from overloading of the cartilage by inadvertent overcorrection into valgus, not from spontaneous degeneration with time.

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