Criteria for Unicompartmental Knee Arthroplasty: Are There Exclusion Criteria?

Fig. 3.1

Intraoperative photo of a unicondylar arthroplasty

../images/469508_1_En_3_Chapter/469508_1_En_3_Fig2_HTML.jpg — Fig. 3.2

Example of current unicompartmental knee arthroplasty design [2]

Age

Traditional criterion for UKA includes age over 60 years old. With the recent popularization of UKA and good results found in increasingly younger patients, the traditional guidance recommending UKA in older patients has slowly changed and age is no longer considered a strict exclusion criterion. For the majority of surgeons, UKA has replaced high tibial osteotomy as the treatment choice for the midrange age population (40–60 years old). Faour et al. noted good results at 12 years in young patients following UKA [4]. Walker et al. looked at patients under the age of 60 retrospectively and noted that those patients were able to return to regular physical activity and that a majority of them were able to reach a higher activity level postoperatively [5]. Streit et al. demonstrated that the Oxford UKA had good results in patients under the age of 60 with an estimated 97% survival at 5 years and only 2% of patients had developed symptomatic arthritis in the other compartments of the knee at 5 years [6].

The thinking on UKA has changed significantly over the last 20 years, as UKA has shifted from an operation well suited for an elderly population, unlikely to develop symptomatic arthritis in other compartments of the knee, to one for young active patient who wish to preserve native knee kinematics.

Weight

While obesity was traditionally thought of as a contraindication to UKA due to the increased compartment pressures, risk of early loosening and the perceived increased risk of progressive degeneration in other compartments of the knee, more recent data suggests that Kozin ’s original exclusion criteria of weight greater than 82 kg may warrant revision.

Obesity, and in particular BMI >40, is nearly universally noted to be associated with increased risks of short-term complications following knee arthroplasty, and particularly of infection. Kandil et al. found that obesity and morbid obesity were found to have significantly increased risk of 90-day complications when compared to nonobese patients [7]. However, Molloy et al. found in a prospective cohort that BMI was not associated with worse outcomes postoperatively. Furthermore, BMI was not found to be a risk factor for loosening at a mean of 10 years, and the best reported outcomes were in patients with BMI in excess of 35 [8]. Plate et al. found that at 24 months, using robotically assisted UKA , BMI did not influence clinical outcomes or readmission rates following UKA [9]. BMI was noted to be associated with higher narcotic analgesic use, as well as increased PT sessions. Van der List et al., in a large meta-analysis, noted no inferior outcomes following UKA in obese patients compared to nonobese patients [10].

The evidence-based shift away from strict exclusion of obese patients warrants further investigation. Furthermore, many factors related to poor outcomes following arthroplasty, which were initially thought to relate to BMI, may actually relate more closely to nutrition status and glycemic control. Further research of these possibly confounding factors will clarify their specific risks for short-, mid-, and long-term failure.

Deformity

While large deformities are still a contraindication to UKA, the accepted degree of deformity has expanded from Kozin and Scott ’s original criteria of <5° of varus and < 5° flexion contracture. Classically, this exclusion criterion was based on the principle of minimal soft tissue releases performed in UKA that limited the surgeon’s ability to correct a large deformity to near-neutral. There was also concern that an under-corrected deformity may lead to increased compartment pressures, aseptic loosening, and early failure rates [11, 12].

In the early 2000s, Scott re-evaluated his criteria [13, 14]. By examining his long-term outcomes over the previous decades, he suggested expanding the criteria to <10° of varus deformity and <5° of valgus deformity (Fig. 3.3). More lenient deformity acceptance largely revolved around the belief that a certain amount of residual varus is acceptable [11, 12, 15]. These outcome studies supported similar long-term survival for up to 7° of residual varus deformity. In a recent article by Kleebald et al., they discussed correcting knees in patients with a large preoperative deformity (>7° of varus), and concluded that patients with large deformities (7–18°) may be considered a surgical candidate for UKA [16]. Similar findings were found for valgus deformity , but to a lesser degree [17].

../images/469508_1_En_3_Chapter/469508_1_En_3_Fig3_HTML.jpg — Fig. 3.3

AP radiograph of knee with severe valgus deformity (> 5°)

ACL Integrity

Since Kozin ’s original article, ACL deficiency has been perceived as a contraindication to UKA due to the inherent alteration in knee kinematics and abnormal contract stresses. While Kozin et al. defined ACL deficiency as an exclusion criterion for UKA , recent literature suggests that ACL deficiency without instability may not be a strict contraindication to partial-knee replacement. Biomechanical data suggests that leveling of the tibial slope may compensate for anterior translation in the ACL-deficient knee without restoring the pivot shift to normal [18].

Engh et al. noted at a mean of 6-year follow-up, that UKA in ACL-deficient knees and that in ACL-intact knees had similar survivorship (93% vs. 94%). Of note, this series excluded patients with clinical instability and only included those with stable knees in spite of their ACL deficiency [19]. Boissonneault et al. showed in a retrospective study that patients undergoing Oxford UKA in ACL-deficient knees had satisfactory results at a mean of 5 years with survivorship comparable to a cohort with intact ACLs [20].

Technical advances and the widening of surgical indications have culminated in the advent of combined UKA and ACL reconstruction surgery , which has shown promising results. In vivo kinematic evaluation of patients following combined ligament reconstruction and UKA demonstrated native knee kinematics i n patients who underwent Oxford UKA and ACL reconstruction [21]. Further, in young active patients, at a mean of 53 months, results of combined surgery showed improved knee society scores, good stability, and no revisions [22].

These recent studies suggest that conventional wisdom is being challenged regarding UKA in patients with ACL-deficient knees. Through careful patient screening, altered intraoperative technique, and via concurrent ligament reconstruction, patients with ACL-deficient knees may benefit from UKA in knees with isolated single compartment disease (Fig. 3.4).