Introduction

Degenerative joint disease is a common diagnosis among adults, with an estimated prevalence of knee osteoarthritis ranging from 4.9% to 16.7%.^1,2 Though difficult to pinpoint exactly, there has been an increase in the prevalence of osteoarthritis among adults in the United States.¹ Patients often present with degeneration of multiple knee compartments, although the process can involve a single compartment. Laskin³ reported that less than 12% of patients in his practice had single-compartment disease and were candidates for unicompartmental knee arthroplasty (UKA). Moreover, isolated lateral compartment arthritis is quite uncommon and has received little attention in the orthopaedic literature. Scott⁴ reported that lateral UKAs constitute less than 1% of all knee arthroplasties. Despite its low prevalence, the aging and increasingly active population ensures that orthopaedic surgeons will manage patients with isolated lateral knee compartment arthritis. For some of these, lateral UKA is a predictable solution.

The cause of lateral compartment knee arthritis is likely multifactorial. Valgus knee deformity, genetic factors, traumatic events, and meniscal pathology may all play a role. Patients present with pain and often mechanical symptoms confined to the lateral aspect of the knee. The physical examination proceeds the same as for any patient with a degenerative knee process. Tenderness along the lateral distal femoral condyle and joint line are often noted. With advanced disease, the valgus deformity becomes more pronounced. Care should be taken to fully examine the knee to ensure the degenerative process does not involve the patellofemoral articulation or medial compartment. Likewise, ligament stability and the ability to passively correct a valgus deformity should be documented. Radiographic evaluation includes a weight-bearing anteroposterior film of both knees, a 45° flexed weight-bearing posteroanterior view, a lateral view, and a sunrise or comparable view of the patellofemoral articulation.

Indications

Orthopaedic surgeons entertain a host of management options for lateral knee compartment arthritis depending on patient characteristics and requirements. These options include nonsurgical modalities, osteotomies, total knee arthroplasty (TKA), or UKA. Our indications for lateral UKA are isolated lateral compartment degenerative changes and symptoms with intact knee ligaments and a passively correctable valgus deformity.

Contraindications

Contraindications include advanced degenerative changes of the medial or patellofemoral compartments, inflammatory arthritic conditions, anterior cruciate or other ligament insufficiency, fixed valgus deformity or deformity greater than 10°, less than 90° of knee flexion, flexion contracture of greater than 10°, or a patient incapable of adhering to a post-arthroplasty lifestyle.^5,6

Discussion

The potential benefits of UKA over TKA are faster recovery with shorter hospitalization and decreased morbidity, improved range of motion, better gait patterns, and bone conservation making future conversion to TKA easier should it be necessary.^7–10 Reports in the literature on medial unicompartmental arthroplasty have shown promising results.^11–14 Extrapolations of these data to the lateral compartment, however, may not be appropriate as it is quite different in terms of anatomic and biomechanical characteristics. There are limited studies that focus on the outcomes of lateral UKA.^5,10,15–18 Marmor¹⁵ was the first to discuss unicompartmental arthroplasty for lateral compartment disease. He reported excellent results in 11 of 14 cases. Ohdera et al.¹⁶ reported good results in 16 of 18 patients with greater than 5 years’ follow-up. Ashraf et al.¹⁷ reported on 88 patients with an average of 9 years’ follow-up and felt that the results of lateral UKA were comparable to those found with medial UKA. Pennington et al.⁵ reported on 29 knees among which there were markedly improved Hospital for Special Surgery knee scores and no revisions at greater than 12 years after lateral UKA. Early and midterm data suggest current-design lateral UKA offers reliable results.^5,10,16,17 As with any operative procedure, careful patient selection and adherence to proper indications and sound surgical technique influence these results. The remainder of this chapter reviews the preparation, surgical approach, technical factors, and postoperative management of lateral UKA.

Setup And Equipment

We utilize the same operating room setup as for a total knee replacement. The patient is positioned supine. A padded bump under the operative hip can be utilized at the surgeon’s discretion. A footrest or sandbag secured to the bed allows the knee to be flexed to 90° and beyond. Others drop the foot of the bed to use a suspended leg technique that allows hyperflexion of the knee.^19–21 A well-padded tourniquet is positioned high on the thigh, with the lower extremity prepared and draped in a manner to allow the surgeon control of the extremity. The procedure is performed under tourniquet for modern cement technique and to improve visualization. Equipment will vary depending on the chosen implant. It is important to have a total knee system available in the event that intraoperative inspection reveals advanced degenerative changes affecting the medial or patellofemoral compartments. Likewise, preoperative counseling should include a discussion of the possibility for total knee arthroplasty. Other considerations include specialized retractors. We like to use a retractor in the notch that extends over the trochlea to protect the patellofemoral compartment. The need for these instruments is dependent on the chosen approach.

 Surgical Approach (see Video 13-1)

The lateral compartment can be easily accessed through a slightly lateralized oblique, extensile, paralateral incision running along the lateral border of the tibial tubercle and the patellar tendon (Fig. 13–1). A modified lateral parapatellar arthrotomy is then performed. There is some concern that this adds complexity in the event that conversion to a TKA is required or that future revision surgery will be more of a challenge. Sah and Scott^6,18 reported this concern and described their technique for lateral UKA through a medial parapatellar approach. To provide another perspective, the lateral parapatellar approach has been quite successful in the senior author’s experience and has not posed problems intraoperatively or in the event of further surgery. The approach we describe provides excellent visualization of the surgical structures and minimizes soft tissue stripping.

Figure 13–1 Paralateral skin incision.

The skin incision is from the superior pole of the patella to just lateral to the tibial tubercle. The length of the incision is adjusted to allow for adequate visualization as needed. Skin flaps are full thickness but not excessively undermined. Once through the skin and subcutaneous fat, the arthrotomy proceeds through the retinaculum, staying lateral to the patellar tendon (Fig. 13–2). A moderate amount of fat is excised under the patellar tendon to allow for adequate visualization in determining tibial rotation. The lateral edge of the tibial plateau is exposed for visualization and to allow proper retractor placement by elevating a small portion of the iliotibial band off of Gerdy’s tubercle. The knee can be flexed and extended to allow visualization of the other compartments through this mobile soft tissue window. Final confirmation of isolated lateral disease is made prior to proceeding. Lateral femoral and tibial osteophytes should be removed. A patellar retractor is carefully placed after inspection of the anterior cruciate ligament to confirm its competence. A lateral Z retractor protects the iliotibial band and the lateral ligamentous and capsular structures after the visible portion of the lateral meniscus is excised.

Figure 13–2 Lateral parapatellar arthrotomy offering excellent exposure of the lateral compartment.

Technical Description