Doc, the Inside Part of My Knee Is Killing Me!

A 63-year-old male presents with approximately 2 years of isolated pain over the medial aspect of his knee. For a prior sporting injury, he underwent an open medial meniscectomy about 30 years ago. Examination reveals a mild varus deformity that does not correct past neutral, isolated medial joint line tenderness, a negative patellofemoral grind test, and a stable ligamentous exam. He has active ROM of 5 to 120 degrees of flexion. He has no significant adjacent joint complaints and has full ROM of his hips, ankles, and feet. The patient reports substantial disability and is looking to return to a more active lifestyle. X-rays of the affected knee are shown in Figure 34-1.

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Figure 34-1. (A) AP and (B) Merchant's view x-rays showing

medial compartment joint space narrowing.

Vignette 34: Answer

When a single compartment of the knee is affected and conservative management is no longer effective, there are 3 basic options for surgery. HTO is reserved for younger patients who are laborers and preferably have intact menisci. UKA is typically for elderly or younger patients not quite ready for a TKA. TKA is a reliable procedure but may be too much when only one part of the knee is diseased. In the setting of isolated unicompartmental arthritis, UKA allows for focal treatment of the disease while retaining uninvolved or minimally involved areas of the knee. In general, the surgery is less invasive and has a quicker recovery than TKA. In our case, the patient is no longer a young laborer with isolated disease, which would favor a UKA over HTO (higher reoperation rates, wound complications, and neurovascular injury) or TKA. Studies have demonstrated that patients with a successful UKA have a more normal-feeling knee, higher satisfaction scores, and 10-year survivorship of approximately 80% to 98%.107,108 _{Most believe that these advantages are}

related to retention of the ACL, and gait analysis studies have demonstrated more normal knee mechanics after UKA when compared with TKA.109

Patients with partial replacements (UKA, patellofemoral arthroplasty, or bicompartmental arthroplas- ties) carry the unique risk of developing progressive arthritis in the unresurfaced compartments. The incidence of this complication has decreased with improvements in surgical technique. Care must be taken to not overstuff the resurfaced compartment and thereby shift forces to the adjacent compartment (overcor- rection of alignment).110 _{It is also important to avoid overhang of the femoral component anteriorly, creating}

patellofemoral impingement.111 _{Despite good overall outcomes and survivorship, most studies and registry}

data note a higher revision rate for UKA compared with TKA.112

Revision of a UKA to a TKA can be straightforward, with results similar to that of primary TKA, or complicated, with results similar to that of revision TKA. Ultimately, the mode of failure and technical issues at the time of the index UKA (particularly the size of the tibial cut) will delegate the complexity of the revi- sion.113-115 _{However, in respecting modern principles of TKA and liberal use of stems and metallic augments,}

UKA conversion to TKA can be performed with a high level of success.

Although indications and contraindications for UKA are constantly evolving (some now suggest it is okay in the ACL-deficient knee and with moderate patellofemoral DJD), the classic criteria were initially described by Scott and Kozinn.45 _{Patients are considered candidates for UKA if they meet the following criteria:}

Disease is isolated to only one compartment.

The patient’s weight is less than 180 lb (BMI < 35 kg/m2_{). Although this is no longer strictly follow-up,}

it is where they will be leading you within a vignette. Obese patients have a higher failure rate, but there is no precise cutoff for BMI or weight.

There is absence of a large coronal plane deformity (< 10 degrees of varus or < 15 degrees of valgus). There is less than a 10-degree flexion contracture.

There is at least a 90-degree arc of motion. There is an absence of inflammatory arthritis. The cruciate ligaments are intact and functional.

The patient has an average activity level and does not have the goal of returning to heavy labor or high-impact sports (typically older than 60 years). When the question writer mentions heavy laborer, he or she is typically pushing toward HTO vs arthrodesis.

The most common UKA designs can be divided into fixed and mobile bearing. The initial UKA designs were fixed bearing and performed poorly.116 _{More recent fixed bearing designs implanted with improved}

surgical indications and technique have demonstrated survivorship of 90% to 98% at 10 years.117 _Mobile-

bearing UKA consists of a polyethylene articular surface that is more conforming to the femoral component with a mobile articulation to a polished tibia baseplate. This provides the theoretical advantage of lower contact stresses with subsequent lower polyethylene wear rates, making this design more attractive for the younger patient population.118 _{Although it is more technically challenging (increased risks for aseptic}

loosening, bearing dislocation, and arthritis progression) to implant than fixed-bearing UKA, specialized centers have reported equivalent survivorship rates of 95% at 10 years.119,120

Why Might This Be Tested? Partial knee replacements have been gaining popularity in the United States

over the past decade. Improved surgical techniques and proper indications are important to understand and are commonly a source of test questions, as are associated complications with UKA.

With isolated unicompartmental disease in the setting of mild deformity and good ROM, UKA is an excellent option. Do not overcorrect the deformity, and remember that a well- done UKA has excellent results whether fixed or mobile bearing in nature.