Medial Unicompartmental Arthroplasty of the Knee

Jason M. Jennings, MD, DPT; Lindsay T. Kleeman-Forsthuber, MD; Michael P. Bolognesi, MD


J Am Acad Orthop Surg. 2019;27(5):166-176. 

In This Article

Implant Options for Medial Unicompartmental Arthroplasty

Mobile- Versus Fixed-bearing Implants

Metal-backed MB implants were introduced in the 1980s and designed to be fully congruent with distribution of contact forces over a large surface area to lower polyethylene wear rates (Figures 2–4).[19] MB implants carry the unique complication of bearing dislocation, which has a variable cited incidence between 0.3% and 4.2%.[19–21] Modern metal-backed FB implant designs have low conformity between the femoral and tibial components to allow for greater ROM and lower backside wear.[22] Conflicting data exist on whether FB or MB implants have lower wear rates. One in vitro study demonstrated superior wear rates for FB designs with linear wear rates of 0.018 mm/million cycles compared with 0.032 mm/million cycles for MB implants.[22] Use of highly cross-linked polyethylene improved wear rates by 68%, but the difference in wear performance between bearing types remained consistent.[22]

Figure 2.

Photograph showing an example of lateral fixed-tibia unicompartmental knee arthroplasty (UKA) bearing (top) and medial mobile-tibial UKA bearing disassembled (bottom).

Figure 3.

Photographs showing the example of mobile-bearing unicompartmental knee arthroplasty (UKA) implant (A) and fixed-bearing UKA implant (B) demonstrating difference in congruency between fixed and mobile constructs.

Figure 4.

Postoperative radiographs of well-functioning left medial mobile-bearing unicompartmental knee arthroplasty, including full-length standing AP (A) and lateral (B) radiographic views.

In vivo data on MB and FB implants have not demonstrated a clear difference in clinical performance between bearings (Table 1). One cohort study by Whittaker et al[23] found higher 5-year survival in the FB group at 96% compared with 89% for the MB group. Parratte et al[19] performed a comparative analysis of FB and MB UKAs at 15-year follow-up and found comparable clinical scores between groups with similar revision rates. A meta-analysis of cohort studies showed lower revision rates attributable to wear for MB implants at 5- and 10-year follow-up; however, no notable difference was found in clinical outcomes, patient-reported outcomes, or overall revision rates between MB and FB implants.[24] A recent systematic review of cohort and registry studies found higher rates of aseptic loosening as a mode of failure in MB implants compared with that in FB implants, but higher rates of osteoarthritis progression in FB implants.[25] Polyethylene wear accounted for 12% of revisions in the FB group and zero in the MB group, whereas dislocation accounted for 11% of failures in the MB group. Overall, the long-term survival seems to be similar between groups, but more focused analysis is needed to identify whether patient factors increase the risk of failure with certain bearings.

Cemented Versus Uncemented Bearings

Cementation of UKA implants can be challenging because of limited visibility and smaller surface area. Aseptic loosening remains the primary indication for revision of UKA. Despite high failure rates with early uncemented UKA designs, renewed interest has been found in uncemented fixation. Kendrick et al[26] randomized patients to either uncemented or cemented Oxford (Zimmer Biomet) implants and found higher incidence of subsidence with uncemented components at 2-year follow-up. The incidence of partial and complete radiolucencies was higher in the cemented group at 62% compared with 29% in the uncemented group. At final follow-up, 24% of cemented implants had complete radiolucency of the tibial component, but all were <1 mm and nonprogressive.[26] Studies have shown no correlation between stable radiolucencies <1 mm and aseptic loosening;[27] however, in the presence of pain, many surgeons would be pressed to revise these implants.[26] Midterm survival for cemented and uncemented implants has shown to be comparable between groups with one study showing 95.4% 10-year survival for cemented UKA and 97.4% for uncemented implants.[28] These results are promising for uncemented UKA, but more long-term follow-up is needed.