In our series of 1000 cemented Phase 3 OUKAs with up to 15 years follow up, lateral progression requiring revision occurred in 25 cases (2.5% cases) at a mean of 7.0 years (range 1.9 to 11.4 years) (Pandit et al, 2015). In this series, cases with lateral progression were matched with controls but no factors leading to progression were identified. In Svard’s 20-year series, the revision rate for lateral compartment OA was 2.3% (Svard & Price, 2006).
Diagnosis
Pain in the knee, usually but not always on the lateral side, is the main symptom. The first radiographic sign is narrowing of the lateral compartment joint space (Fig. 11.11), and this may long precede the onset of pain. Subchondral sclerosis and disappearance of the joint space ensue. Osteophytes around the margins of the lateral compartment are very common but do not necessarily portend progressive arthritis.
Figure 11.11 Severe lateral compartment degeneration.
Causes
Some authors have regarded arthritis of the contralateral compartment in UKA as a time-dependent consequence of the gradual, but inevitable, spread of osteoarthritis throughout the joint (Dejour et al, 1998 ), perhaps hastened by the presence within the joint cavity of the foreign materials of the prosthesis. If this were true, there would be evidence of progressive arthritis in both the lateral and PFJ. A comparison between one year and ten year post-operative radiographs shows that this does not happen (Weale et al, 1999). Furthermore the incidence of revision due to progression is rare (about 2.5% at 15 to 20 years) (Liddle et al, 2013; Svard & Price, 2001) and revision for PFJ progression virtually never occurs. It would seem that, in general in AMOA, disease progression is arrested by OUKA (Svard & Price, 2001) suggesting that the underlying disease is a focal mechanical problem. In the few cases when progression does occur, it is likely to relate to some issue with the indications or technique or perhaps an element of inflammatory arthritis or a low grade infection.
Most authors believe, as we do, that overcorrection of the varus deformity into valgus (Fig. 11.12) is an important cause, and many surgeons recommend aiming to leave the UKA knee in a few degrees of varus to avoid this. Choosing the postoperative tibiofemoral angle is not an option in the OUKA operation since the thickness of the bearing is selected to match the lengths of the ligaments, not to provide an arbitrary alignment of the limb. Therefore, an intact MCL is all-important if overcorrection is to be avoided.
Figure 11.12 Postoperative radiograph of a knee overcorrected into valgus. Such a degree of valgus could only occur after damage to the MCL.
To minimise the risk of lateral compartment OA, we recommend valgus stress radiographs to assess the lateral compartment pre-operatively, very careful intraoperative preservation of the MCL (particularly its deep fibres) and avoidance of ‘overstuffing’ of the medial compartment with a thicker bearing than the ligaments will easily accommodate.
Treatment
If the symptoms warrant surgical treatment, revision to TKA is indicated. However, experienced surgeons may chose to do a lateral UKA if the medial compartment is satisfactory. We tend to open and extend the old incision and then do a lateral parapatellar approach. In our series of 27 cases with a mean follow up of four years, we have had no failures (Dodd 2014). Furthermore we find that patients recover more rapidly (length of stay 2.3 days vs 4.7 days) and with less morbidity and better functional outcomes than after TKA.
