2.1 Stage of osteoarthritis

An osteotomy is a biological procedure which aims to shift peak load areas from the medial compartment to central and lateral areas. The best results are obtained in limited chondral defects on the medial side and the outcome will be compromised the more the osteoarthritis has progressed. The patient should be informed that limited pain relief must be expected if there is already 4th degree osteoarthritis on the medial side with relative medial instability [1]. HTO is not indicated in pagoda-type tibiae, meaning severe bone loss on the medial side and a sloped lateral compartment ( Fig 3-2 ). It is very difficult to choose the correction angle in this situation, and failures by under-or overcorrection are common. Unicompartmental knee replacement is advisable in this situation. Obviously HTO is not indicated after substantial lateral meniscectomy and in severe lateral osteoarthritis. MRI scans should not be relied upon in decision-making, since the sensitivity and specifity for chondral defects are low. Arthroscopy tends to overestimate chondral pathology on the lateral side. Softening of the tibial chondral surface is a normal finding in adults and no contraindication against HTO. Superficial fraying of the femoral cartilage is not relevant and can be ignored. Important findings are defect zones in the load-bearing areas and ruptures or deficiencies of the lateral meniscus. We rely more on stress x-rays in questionable cases and consider significant closure of the lateral joint side under valgus stress as an exclusion criteria for HTO and also for a UKA ( Fig 3-3 ).

2.2 Patellofemoral joint

Many patients with medial joint pain have degenerative changes in the patellofemoral joint as well. If the clinical symptoms are clearly those of medial osteoarthritis, these changes can be ignored in the decision-making process and should not guide the surgeon towards a TKA. Certainly the patient has to be informed that stair climbing or downhill walking may be compromised after the procedure but the leading symptoms of joint-line pain will be cured, as in patients without patellofemoral joint pathology. In open-wedge HTO it is advisable to use the modified biplanar technique with the anterior osteotomy plane sloped downwards (see chapter 15 “Rotational osteotomies of the tibia and femur”, Fig 15-4 ). This modification avoids patella infera and rules out pressure increases in the patellofemoral joint [2, 3]. Current literature indicates that at least mobile bearing UKAs can be safely implanted in patients with patellofemoral degeneration without increasing the middle- and long-term revision rate [4–6].

2.3 Ligamentous status

HTO has a wide indication range in patients with instable knees and is a fundamental part of the therapeutic repertoire. The common combination of persistent instability, meniscectomy, and medial osteoarthritis in patients with varus morphotype can be treated ideally by an open-wedge valgus/extension osteotomy of the tibia. The combination of posterior/posterolateral instability and varus morphotype requires a flexion/valgization osteotomy (see chapter 11 “Osteotomy and ligament instability: tibial slope corrections and combined procedures around the knee joint”). The only contraindication for HTO would be a significant deficiency of the medial collateral ligament (MCL) with risk of secondary ligamentous valgus, a situation the author has encountered very rarely. In contrast, in many cases of preexisting MCL injuries the ligament can be retensioned by open-wedge osteotomy if the distal part is not detached during the procedure (see chapter 16 “Total knee arthroplasty after osteotomy around the knee”, Fig 16-8 ).

On the other hand, the correct function of a UKA is strongly dependent on an intact anterior cruciate ligament (ACL). The revision rate is unacceptably high if a UKA is implanted in an ACL deficient knee [5]. In this respect, it is important to understand the morphological differences between medial osteoarthritis in ACL-intact and ACL-deficient knees. If the ACL is intact, the relative position of the tibia on the femur is constant and the osteoarthritis is obligatory anterior on the tibia and distal on the femur which corresponds to the areas with highest physiological load. Since the posterior part of the femur and the tibia will still have a chondral surface, the deformity is restricted to the extended and slightly flexed position of the knee and will reduce completely in flexion. The MCL will be slack in extension due to the wear, and tight in flexion because the intact chondral surfaces retension the ligament.

In ACL deficiency, the tibia will shift to an anterior position relative to the femur. The contact point shifts posterior on the tibial plateau and the osteoarthritis will develop posteromedially, often resulting in a dished-form defect of the posteromedial tibia (cupula). At this stage the anterior subluxation of the tibia is fixed and cannot be reduced anymore, meaning that the clinical instability may appear less obvious, although the ACL is completely deficient.

Knowing these mechanisms, the surgeon may rule out cases for UKA simply by carefully assessing the plain lateral x-rays ( Fig 3-4 ), whereas the indication for a HTO may still be given even in chronic ACL deficiency.

If there is concern about the stage of osteoarthritis especially in the lateral compartment, we recommend performing stress x-rays with manual or instrumented varus and valgus stress. If the lateral joint space closes under stress, neither HTO nor UKA are indicated any more, and a total knee replacement is necessary. If the narrowed medial joint space does not open to the regular width in 20° knee flexion, a contracture of the MCL is present which rules out the typical situation of anteromedial osteoarthritis (see Fig 3-3 ) and a UKA should not be implanted. For the same reason, a UKA is also not indicated if the osteoarthritis involves the entire medial tibia on a lateral x-ray. In these cases the degeneration has either progressed to general osteoarthritis or is correlated with chronic ACL deficiency and will not respond sufficiently to monocompartmental arthroplasty.