Introduction

Unicompartmental knee arthroplasty (UKA) has gained popularity in the past decade. The concept of unicondylar knee replacement was introduced in the 1970s by Marmor. He reported pain relief in 86.6% of his patients at an average of 11 years of follow-up with this prosthesis.¹ However, Insall and Aglietti² and Laskin³ reported high failure rates of 26% and 20%, respectively, especially with medial compartment arthroplasty. Most of the failures in subsequent reviews have been attributed to inappropriate patient selection, technical errors, and poor implant design.¹ O’Rourke et al.⁴ recently reported a survivorship of 96% at 5 years and 84% at 20 years of follow-up with the Marmor prosthesis. Over the years, refinement in the surgical technique and improved implant designs and instrumentation have led to the resurgence of interest in UKA. The purported advantages of UKA over proximal tibial osteotomy are better pain relief, quicker return to function, fewer complications, and better long-term results. Furthermore, compared to a total knee arthroplasty (TKA), UKA has a more physiologic gait and better range of motion as the cruciate ligaments, contralateral menisci, and other structures are preserved.⁵ Also, the recovery is faster⁶ and it can be easily revised to a TKA with minimum morbidity if required for failure.

Since 2002, the senior author has been using a reproducible and accurate minimally invasive surgical technique (modified slightly from that described by Gesell and Tria ⁷), with a cemented modular fixed-bearing Miller-Galante (MG) implant (Zimmer, Warsaw, IN), which is described in detail in this chapter. Naudie et al.⁸ reported a 94% survivorship at 5 years and 90% at 10 years with the MG prosthesis. Argenson et al.⁹ also found survivorship of 97% at 5.5 years of follow-up with the same implant. Berger et al.^10,¹¹ reported Jorge Galante’s MG survivorship of 98% at 10 years and 95.7% at 13 years of follow-up. Minimally invasive surgery (MIS) has evolved over the years from more traditional UKA with the use of better instrumentation and modified surgical technique. MIS philosophy does not mean a small skin incision but aims at bone-sparing technique and preservation of all possible soft tissues. Also, during arthrotomy and surgery the quadriceps tendon is not incised and the patella is not averted, thereby preserving the integrity of the quadriceps mechanism.¹² MIS allows for a faster, less painful recovery, shorter hospital stay, and better cosmesis. Attempts to perform MIS surgery with a careless surgical technique can lead to component malposition, malalignment, and significant soft tissue damage. Coon¹² found that short-term results of MIS UKA were comparable to that of traditional UKA. Pandit et al.⁶ also found survival rates of 97.3% at 7 years with MIS technique. Since failures if they occur are more common early on, Pandit et al. believed that results of MIS should be comparable to the traditional UKA in the long term.

Preoperative Evaluation

One of the most important requirements for a successful outcome of UKA is patient selection. Kozinn and Scott ¹³ summarized their patient selection criteria in their UKA review paper from 1989. Those that we consider most important are as follows.

The ideal candidate for unicondylar knee arthroplasty is a low-demand, thin patient (>55 years of age) with isolated medial unicompartmental disease. The range of motion of the knee joint should be greater than 90° and a flexion contracture, if present, should be 5° or less. The valgus deformity should be less than 15°, and varus less than 10° should be passively correctable to neutral. There should be no rest pain or evidence of inflammatory arthropathy. There should not be diffuse pain or significant involvement of other knee compartments. The lateral compartment should have full-thickness joint space on the anteroposterior radiograph and also well-preserved meniscus and articular cartilage. Patellofemoral pain is a relative contraindication. The knee should be stable without any insufficiency of the cruciate or collateral ligaments. Medial or lateral subluxation or posterior tibial bone loss is suggestive of crucial ligament insufficiency. Patients with osteonecrosis of a single femoral condyle should not have extensive metaphyseal-diaphyseal involvement. Ideally the patient should be able to pinpoint the medial compartment as the source of pain, which worsens significantly with weight bearing (the so-called one-finger test).¹⁴

A thorough history, physical examination, and radiographic examination are done for all patients. Radiographic examination is of the utmost importance for medial UKA planning. The patient should have anteroposterior standing weight-bearing radiographs of the knee in extension and 45° of flexion (the so-called tunnel view) on a large film to judge the mechanical alignment. There should be no tibiofemoral subluxation or translocation, which denotes involvement of the contralateral compartment. A skyline view should be done to assess the patellofemoral compartment. The lateral view is helpful to determine the posterior tibial slope, look for any posterior femoral osteophyte that might impinge in deep flexion, and determine if the tibial wear pattern is excessively posteromedial (which could connote anterior cruciate ligament [ACL] insufficiency). A notch view or tunnel view (anteroposterior view with flexed knee) also helps in evaluation of cartilage thinning, which is much more impressive in flexion. Magnetic resonance imaging is required in some cases of osteonecrosis to evaluate the extent of involvement. It may also help in evaluating the other compartments’ and the cartilage thickness.

Surgical Technique for Medial Fixed-Bearing Onlay UKA

Positioning and Draping

We prefer that UKA patients undergo regional anesthesia unless contraindicated. The anesthesia team should be aware of the fact that the patient will be required to walk and start with the physical therapy soon after completion of surgery. Care is taken not to give drugs that might overly sedate patients for many hours. Patient preparation is done as per standard protocols. The patient is positioned supine on the operating table. The tourniquet is placed high up on the thigh so flexion contracture can be more easily seen, and an impervious drape is applied to prevent wetness below the tourniquet that could damage the skin. The patient operative site shaving is done just before prepping with a safety razor. A prescrub with chlorhexidine soap and water is done and the skin is prepped with tinted chlorhexidine solution. The skin of the extremity is draped with iodine-impregnated plastic draping to make the operative field impervious and decrease skin flora. Also, the MIS approach requires continuous repositioning of the knee to different degrees of flexion to optimize visualization, so care is taken to drape the extremity free with slackness in the drapes. We prefer a bump positioned at 90° of flexion, but a leg holder can be equally effective.

Approach

The skin incision (Fig. 17–1) is slightly medial to the midline, extending from the superior edge of the patella to the medial aspect of the tibial tubercle. The incision is approximately 10 cm long depending on patient size and adipose tissue, and can be extended to avoid applying too much tension to skin during retraction. A “mobile window” is created by releasing adipose tissue from the retinaculum approximately 2 cm circumferentially around the incision. A medial parapatellar arthrotomy (Fig. 17–2) is performed from a level 1 fingerbreadth above the superior pole of patella to a point approximately 15 mm distally to the tibial joint line. The arthrotomy does not damage the vastus medialis muscle. Also, the technique does not require eversion or dislocation of the patella, thus preserving the suprapatellar pouch. This may reduce postoperative pain and allow faster quadriceps control, since disruption of the extensor mechanism is minimal.