Introduction

Unicompartmental knee arthroplasty (UKA) has become a common surgical procedure for unicompartmental osteoarthritis both nationally and internationally in the middle-aged and older patient. Midterm and long-term survivorship of UKA is highly variable. From 1998 to 2005, the incidence of performance of UKA increased at a rate of 32.5% compared to an increase in the performance of total knee arthroplasty of 9.4% during the same period.¹ In the past 30 years, multiple authors have reported on the indications and contraindications for UKA.^2–23

Indications

The classic indications for UKA include:

The “one-finger test” has been reported as a useful indicator as to the location of the pain correlative with the patient’s radiographs exhibiting single-compartment osteoarthritis.³

Contraindications

Contraindications have included:

Operating on patients who exhibit contraindications can result in reduced survivorship of UKA.^2,3

Etiology of Failures

Furthermore, failures of UKA can occur as a result of:

Some designs that are no longer utilized exhibited exceedingly high failure rates as a result of component design and polyethylene failure.^24,25 Furthermore, using all-polyethylene tibial (APT) components in elderly patients with increased posterior tibial slope and removal of significant amounts of tibial bone have resulted in increased medial tibial collapse with subsidence of the tibial component in some patients.²⁶ Despite recent articles indicating that metal backing reduces strains across the cement-bone interface,²⁷ the longest published survivorship has utilized APT components with specific undersurface macrostructure on both the inferior portion of the APT component and its eccentric post. However, it has been emphasized that minimal tibial bone was removed in those cases that exhibited the longest survivorship.^19,28

 The Importance of Surgical Technique (see Video 24-1)

Surgical technique is critical when performing UKA, and accounts for failures regardless of the prosthesis utilized. Surgical recommendations include lateralization of the femoral component when doing medial UKA to account for external rotation during extension of the knee (Fig. 24–1). When using an all-polyethylene component, it is important to implant the widest tibial component possible from the medial tibial cortex to the anterior cruciate ligament, resting on the medial tibial cortex (Fig. 24–2). Using a Steinmann pin through the intercondylar notch to retract the patella and facilitate exposure is helpful. Appropriate cement technique is critical and involves drilling, using pulse lavage, and drying the surfaces prior to applying the cement (Fig. 24–3). The cement should be applied in its doughy stage as well.²⁹ This allows for improved fixation of both the femoral and tibial components.³⁰ When choosing APT components, laboratory studies have confirmed that a dovetail undersurface macrostructure of the tibial component provides the greatest resistance to lift-off and shear stresses (Fig. 24–4). Waffle undersurface patterns tend to result in the worst resistance to lift-off and shear stress application.^31,32

Figure 24–1 Lateralization of femoral component in medial UKA.

Figure 24–2 APT tibial component resting on medial tibial cortex.

Figure 24–3 Drilling of femur prior to cementing status post pulse lavage and drying.

Figure 24–4 Dovetail interface structure on undersurface of tibial APT component.

Complications of Surgical Technique

It is critical to avoid removal of too much tibial bone in order to prevent fracture and/or subsidence of the tibial component. Fractures have been reported (Figs. 24–5 and 24–6) through the tibial pin sites, resulting in difficult revision scenarios.^33–35 Early medial collapse has been reported with significant medial bone removal, and an 11% rate of subsidence of the medially placed APT components has been reported with 2.5-year follow-ups. Upon radiographic review of these patients, the tibial component lacked peripheral support and there was significant bone removal.²⁶ As originally reported by Bloebaum et al.,³⁶ the majority of the proximal tibia, when resecting approximately 6–11 mm below the articular surface, is highly porous cancellous bone consisting predominantly of air. When viewing these resected specimens through the electron microscope, it is apparent that up to 80% of the proximal tibia is cancellous bone at these levels, as demonstrated by polarized (Fig. 24–7A) and nonpolarized (Fig. 24–7B) electron microscopic views of the proximal tibia.³⁶ Furthermore, the posteromedial and posterolateral aspects of the resected proximal tibia have up to 20% more bone than the remainder of the articular surface. This observation would clearly explain why fixation can be an issue and subsidence may occur, especially anteromedially and anterolaterally, if too much tibial bone is resected or an undersized tibial component is placed without medial tibial cortex contact and support.

Figure 24–5 Fracture of tibia medially status post pin placement.

Figure 24–6 Catastrophic fracture of tibia status post lateral UKA.

Figure 24–7 Electron microscopic polarized view (A) and nonpolarized view (B) of proximal tibia 8–11 mm below the articular surface.

Malalignment

Alignment is critical to the success of UKA. Squire et al. originally reported this concept in 1999 when they noted in a series of UKA patients that 46% had loss of lateral joint space postoperatively.³⁷ The etiology of lateral compartment wear subsequent to medial UKA is usually secondary to overcorrecting preoperative varus alignment and placing the patient into postoperative valgus (Fig. 24–8). In one consecutive series of 100 patients,¹⁶ 83% developed progressive lateral joint space collapse due to progressive wear of the lateral compartment (Fig. 24–9). Berger et al.,²³ however, noted that only 18% developed progressive joint space collapse subsequent to UKA when these patients were placed in neutral to slight mechanical varus (Fig. 24–10). Biopsies of the pristine surface of the lateral compartment in “normal”-appearing lateral compartment chondral surfaces has confirmed early changes of osteoarthritis histologically, indicating that preexistent degenerative arthritis may be present in asymptomatic knee compartments.³⁸ Medial-lateral mismatch can occur due to inappropriate placement of the components in the sagittal plane on the medial femoral condyle or the tibial plateau. In addition, sizing of the femur and tibia is important as well since the tibial component should rest on the medial tibial cortex to avoid subsidence, which is a common failure modality if too much tibial bone is removed or the tibial component is undersized. Lateral positioning of the femoral component onto the medial femoral condyle is helpful to avoid medial-lateral mismatch.^3,25,26 Older prosthetic designs more commonly resulted in articular surface mismatch due to attempts at matching surface geometry configurations of the femoral and tibial components (Fig. 24–11).

Figure 24–8 Severe valgus malalignment status post UKA.

Figure 24–9 Progressive lateral joint space collapse 10 years status post medial UKA.

Figure 24–10 Postoperative UKA placed in neutral to slight anatomic varus.

Figure 24–11 Medial-lateral mismatch of femoral and tibial components.

Failure of Fixation

Failure of fixation is most commonly due to cement technique and/or component design (Fig. 24–12). Polyethylene wear with premature failure of the tibial insert was more common previously due to the usage of thin polyethylene inserts (Fig. 24–13) and sterilization issues resulting in delamination and early wear of the polyethylene tibial insert.²⁵ Cement removal can be compromised due to a mini-incision exposure resulting in cement retained posterior to the APT component. When implanting the tibial component, a “snowplow effect” (Fig. 24–14A) can occur resulting in “heaping up” of bone posteriorly (Fig. 24–14B and 14C), which can eventually fracture and result in loose body formation.³⁹ It is important to use a small curved curette or similar instrument to remove cement posteriorly when implanting the APT component.

Figure 24–12 Failed fixation of UKA.

Figure 24–13 (A) Failed polyethylene insert exhibiting delamination, pitting, fracture, and wear. (B) Failed polyethylene insert exhibiting severe wear.

Figure 24–14 (A) Snowplow effect illustrating “heaping” of snow. (B) Cement buildup posterior to tibial component. (C) Cement “loose body” occurring 2° to fracture of built-up cement posteriorly.

Conclusions

In summary, failures of UKA are secondary to poor surgical technique resulting in tibial fracture, and medial-lateral mismatch with resultant component maltracking and loosening. Other causative factors include poor cement implantation technique as a result of ignoring the concepts of drilling, pulse lavage, and drying the bony cut surfaces prior to cementing the components. Furthermore, removal of too much tibial bone may result in component subsidence due to the significant porosity of bone occurring in the proximal tibia. Poor component design has contributed to medial-lateral mismatch, articular surface incongruity, and mobile-bearing dislocation. Component and/or limb malalignment may occur if the surgeon is not consistently monitoring alignment during the surgical procedure. Inappropriate patient selection is perhaps the most critical component resulting in prolonged survivorship for UKA. Patients with generalized knee pain tend to get incomplete relief of pain from a unicompartmental replacement. Extremely active patients tend to have earlier failure rates than those who are able to avoid significant activity levels. Obesity logically should be a contraindication to long-term survivorship for this procedure; however, midterm survivorship studies have been unable to substantiate this theory. If a well-designed UKA component system is implanted correctly in the appropriately chosen patient, UKA is a highly successful procedure in the middle-aged or older patient. As long as minimal tibial bone is removed at the time of the index procedure, revision total knee arthroplasty procedures are extremely successful for failed UKA and not difficult to perform.