8.4 Osteotomies around the knee

Osteotomy of the proximal tibia or distal femur can alter the alignment of the knee in three planes—coronal, sagittal, or rotational axes.

The aim of the osteotomy might be to correct deformity due to fracture, growth plate arrest, or metabolic disease; to unload a compartment damaged by osteoarthritis or cartilage injury or to improve stability (Box 8.4.1).

The ideal patient will be active, young or middle aged, and have healthy bones and a mobile knee. The pathology will be either a unicompartmental problem or involve instability with deformity.

The principles of osteotomy involve creating the osteotomy as close to the site of the deformity as possible, preferably through cancellous bone with stable internal fixation (Box 8.4.2).

The principle of osteotomy to unload a damaged compartment is based on the mechanical pathogenesis of osteoarthritis.

The mechanical axis (hip/knee/ankle alignment) of the leg is 0 degrees although the anatomical axis (femoro–tibial angle) is 5 degrees of valgus. This results from the distal femur being 8 degrees of valgus and the proximal tibia being 3 degrees of varus. With this alignment, 70% of the weight-bearing load passes through the medial compartment.

There are a number of factors which may increase this load, such as obesity. With an intact medial meniscus the surface contact area between medial femoral condyle and medial tibial condyle is 6cm². Total meniscectomy reduces this to 2cm² and hence a three times increase in load per unit area.

Articular cartilage is designed to withstand loads of 25kg/cm²—if the load exceeds 35kg/cm² then matrix breakdown begins.

Cadaver studies have shown that in normal joints, very small changes in the relative alignment of the femur and the tibia produced marked changes in the amount of weight borne by the medial and lateral compartments of the knee. Osteotomy for osteoarthritis is based on this unloading effect on the damaged compartment.

Soft tissue surgery for correction of collateral instability is unlikely to be successful in the presence of malalignment with tension forces stretching the ligament repair or reconstruction. For example, lateral ligament instability will be aggravated in a varus knee. Corrective valgus osteotomy will relieve stress on the lateral soft tissues.

In a study relating gait analysis to results of valgus high tibial osteotomy (HTO), a low adduction moment gave uniformly good results whilst patients having a high adduction moment (varus thrust on walking) had only 64% good results.

With collateral ligament instabilities, balance of the knee is improved by osteotomy which moves the mechanical axis to the convex side. This relieves the tension stresses on the side of laxity although soft tissue repair or reconstruction is often also required to achieve stability (Figure 8.4.1).

The normal posterior slope of the medial tibial condyle is 6.7 degrees and the lateral tibial condyle 3 degrees. Any increase in slope will contribute to anterior translation of the tibia (and aggravate anterior cruciate ligament [ACL] instability) whilst a decrease slope will contribute to posterior translation (Figure 8.4.2). Any osteotomy that alters tibial slope may therefore influence cruciate stability.

Opening wedge HTO tends to result in increased tibial slope and thus increased anterior tibial translation. This is an advantage in posterior cruciate ligament instability resulting in symptomatic improvement. The converse is true of closing wedge HTO where tibial slope tends to decrease, favouring its use in the ACL deficient knee.

Opening wedge HTO increasing the posterior tibial slope is also indicated in situations of hyperextension caused by premature closure of the anterior tibial growth plate or in some cases of soft tissue instability.

With varus plus ACL instability there is debate as to whether ACL reconstruction alone is sufficient to control symptoms or whether combined HTO and ACL reconstruction is more reliable. ACL graft forces are high in varus and internal rotation and most surgeons favour combined surgery (Figures 8.4.3–8.4.5).

In the young patient with osteochondritis dissecans or post-traumatic full thickness articular cartilage loss on weight-bearing aspects of the femoral condyles consideration may be given to cartilage repair procedures such as microfracture, autologous chondrocyte implantation, or osteochondral grafting (Figure 8.4.6). Mechanical axis alignment becomes important as such procedures are unlikely to succeed if subject to excessive load. Hence osteotomy may be considered to unload a medial femoral condyle lesion in a varus knee or lateral lesion in a valgus knee. However, to reduce pressure on articular cartilage soft tissue release may also be required.

Meniscectomy predisposes to the premature onset of osteoarthritis, especially in association with malalignment (Figure 8.4.7). A long-term follow-up of arthroscopic lateral meniscectomy has shown a very high incidence of osteoarthritis.

When assessing the patient with postmeniscectomy pain, alignment is important. Magnetic resonance imaging or bone scan may confirm excessive stress in subchondral bone with thinning of articular cartilage and osteotomy may be appropriate to unload the damaged compartment.

Replacement arthroplasty of the knee involves a polished metal on ultra-high-molecular-weight polyethylene articulation. Polyethylene has a load to failure of 25MPa and this is exceeded in all high-impact activities which result in subsurface cracks, delamination, and fragmentation. Activities such as hiking and jogging also exceed polyethylene yield points.

As a result, the young, heavy, and active patient with osteoarthritis may be best managed with osteotomy whilst the older, lighter, and less active patient might be better managed with an arthroplasty. Comparative studies between tibial osteotomy and medial unicompartmental replacement have tended to favour arthroplasty, underlying the importance of patient selection for the appropriate procedure.

Osteotomy may be considered a four-dimensional procedure involving:

Active and passive range of movement is assessed noting fixed flexion or fixed angular deformity. The stability of the soft tissues and the mechanical axis alignment when weight bearing is recorded. The gait pattern is observed, especially noting any adduction moment or varus thrust.

Standard anteroposterior, lateral, intercondylar, and skyline views are obtained for basic assessment.

A full-length standing hip/knee/ankle anteroposterior x-ray is required to assess the mechanical axis (Figure 8.4.8). Single leg standing x-rays will show greater deformity if a varus thrust is present on walking.

On occasion, varus valgus stress views are of value to assess soft tissue tension and determine whether soft tissue release is necessary for decompression or correction of alignment.

A bone scan may be helpful to demonstrate increased uptake in over loaded bones.

Determination of the desired position of the mechanical axis is critical to allow templating with measurement of the required angle of correction. In the varus osteoarthritic knee this is commonly at Fujisawa’s point at 35% across the lateral tibial condyle. The degree of correction may also vary depending on the degree of joint damage with more correction required for greater degree of cartilage loss (Figure 8.4.9).

The amount of correction required to place the mechanical axis in the desired position and the tibial slope correction is measured using x-ray templates (Figure 8.4.10).

Intraoperative instrumented jigs allow measurement of the angular correction during surgery.

If the accuracy of computer navigation in osteotomy can be confirmed then it holds great promise for more accurate correction of deformity which should lead to improved results. The essential advantage of computer assisted surgery is accurate localization of the femoral head during surgery which should improve the accuracy of assessment of the mechanical axis.

Most commonly bone loss is on the tibial side and proximal tibial osteotomy is appropriate. This may be opening wedge from the medial side or closing wedge from the lateral side.

A lateral approach involves reflection of tibialis anterior to expose the lateral surface of the tibia. A laterally-based wedge of bone immediately proximal to the tibial tubercle is excised. To allow closure of the osteotomy, release of the fibula is required and this may be performed at different levels. Release through the superior tibiofibular joint is safest and most popular; osteotomy of the fibular head or neck places the common peroneal nerve at risk; through the proximal third of the fibula threatens the nerve to extensor hallucis longus; and through the distal third of the fibula has a high incidence of non-union.

For stability of the osteotomy it is important to protect the medial tibial cortex and maintain correction with internal fixation.

The advantage of closing wedge HTO is bone-to-bone apposition which allows for early weight bearing and bone union. It is therefore favoured in older patients with osteoarthritis, large corrections of more than 12 degrees, and with associated ACL instability to reduce the posterior slope of the tibia.

A medial approach releasing part of the superficial medial collateral ligament is performed and the osteotomy is directed proximal to the tibial tubercle towards the superior tibiofibular joint. Again the opposite cortex is preserved intact for stability. Gradual opening to the desired correction and stable fixation is required. For larger corrections bone graft is recommended—this can be autograft, allograft, or synthetic bone graft substitute. Major risks are fracture into the knee joint and non-union.

The advantages are that the fibula is not disturbed and there is less distortion of proximal tibial morphology which may be important should total knee replacement be required at a later date.

Opening wedge osteotomy is therefore recommended in younger patients, for corrections of less than 12 degrees or for posterior cruciate ligament instability or hyperextension because of the increased posterior slope of the tibia.

In valgus knees the bone loss contributing to the deformity is most commonly on the femoral side (Figure 8.4.11). Hence a distal femoral osteotomy is recommended. As with the tibia this can be opening wedge or closing wedge but risk of non-union has led to closing wedge being more reliable. Again accurate intraoperative correction is required; protection of the opposite cortex and stable internal fixation to maintain correction, promote bone union, and allow early motion.

In some circumstances, e.g. previous lateral meniscectomy or fractured lateral tibial condyle, more minor valgus deformities may be corrected by a varus closing wedge HTO. However the correction should not exceed 8 degrees because of risks of creating an oblique joint line.

Hyperextension or recurvatum deformity may follow premature closure of the anterior tibial growth plate, soft tissue instability, or fracture malunion (Figure 8.4.12).

Anterior opening wedge HTO and bone grafting will correct the deformity (Figure 8.4.13). If proximal to the tibial tubercle there is risk of increased patellofemoral pressure.

There is a place for both opening wedge and closing wedge HTO but on the femoral side closing wedge is favoured because of the high incidence of delayed union and non-union with opening wedge distal femoral osteotomy (Table 8.4.1, Figures 8.4.14 and 8.4.15).

The risks of HTO are summarized in Box 8.4.3.

In general, osteotomy has been shown to improve function and provide pain relief for the majority of patients. A 10- to 21-year follow-up study on tibial osteotomy for varus osteoarthritis demonstrated that osteotomy allowed 10 to 15 years of pain relief, good range of motion and function in a large number of patients. However, results tend to deteriorate with time.

The 10-year survival rate of HTO varies from 51% to 90% in different studies.

In a Cochrane review (2007) based on 13 studies, most studies showed less pain and improvement in function scores after osteotomy, but no study compared osteotomy with conservative treatment.

Function can be improved with low-impact activities such as walking and stair climbing but improvement in true activity levels as measured on the Tegner scale has not been demonstrated.

Inadequate symptomatic relief with persisting pain and limited function often associated with recurrent varus deformity or complications may require revision to total replacement arthroplasty.

A number of studies have reported poorer results in the osteotomy group with reduced flexion and higher reoperation rate with many concluding that total knee arthroplasty (TKA) after HTO is a technically more challenging procedure than primary TKA. The functional outcomes at a mean follow-up of 5 years after TKA in patients with a previous HTO tend to be inferior.

Others have reported a high rate of radiographic evidence of loosening with 8% of knees revised at 5.9 years postarthroplasty. Male gender, increased weight, young age at the time of TKA, coronal laxity, and preoperative limb malalignment have been identified as risk factors for early failure.

In contrast, a comparative study of patients who had bilateral total knee replacement following unilateral HTO reported that while patients with a previous HTO have important differences preoperatively, including valgus alignment, patella infera, and decreased bone stock in the proximal part of the tibia, results of arthroplasty in knees with and without a previous HTO are not substantially different.

Perceived difficulties are listed in Box 8.4.4.

The principles for revision of osteotomy to arthroplasty involve:

In conclusion, most authors agree that it is technically more demanding to perform a total knee arthroplasty after an HTO.

Osteotomy about the knee is an effective method of unloading a damaged compartment and improving stability. Surgery for ligament reconstruction or cartilage repair will fail in the presence of malalignment. However, the aims and principles of osteotomy must be adhered to, together with careful preoperative preparation and surgical technique.

Maintenance of correction of the osteotomy correlates with long-term successful outcome.