While aseptic loosening and osteoarthritis progression are the most common causes of failure after UKA, bearing failure in the form of dislocation or fracture is not uncommon and occurs in about 11% of patients [8, 9]. Bearing dislocation, which is more commonly reported in mobile-bearing UKA, can occur due to a multitude of causes, including residual knee deformity, collateral ligament insufficiency, component malpositioning, impingement, repetitive deep knee flexion or infection . Regardless of whether the bearing dislocation was traumatic or atraumatic, patients who experience this complication after UKA often have predisposing conditions that are multifactorial in nature. For instance, Kawaguchi et al. reported a case of meniscal bearing dislocation in a patient who rolled over while sleeping . The cause of failure in their case report was hypothesized to be a combination of small femur, small tibial component, and valgus knee deformity. Apostolopoulos et al. reported a case of atraumatic bearing dislocation 5 years after UKA . The cause of failure in their report was hypothesized to be aseptic loosening leading to metallosis and bearing instability. Similarly, in our case report, we described a patient who sustained an atraumatic bearing dislocation after months of chronic instability. The intraoperative evidence of metallosis and the patient’s chronic pain preceding the dislocation event suggest that the patient suffered from an attenuation of collateral ligaments that led to chronic instability and wear that precipitated the bearing dislocation.
This multifactorial nature of a bearing dislocation in UKA is important to recognize because revision surgery can result in a high failure rate and re-dislocation if all the predisposing factors for bearing dislocation are not addressed at the time of revision surgery. For instance, Kim et al. reported re-dislocation in 21% of the patients treated with a simple bearing exchange in their retrospective study of 1576 patients who underwent UKA [7, 13]. This high rate of dislocation was hypothesized to be due to failure to address the initial cause of bearing dislocation. Therefore, as a general rule, revision to TKA is recommended in patients with incompetent collateral ligaments, significant coronal or sagittal plane deformity, progression of arthritis in the other compartments, or chronic prosthetic joint infection . However, bearing exchange is a reasonable option where the underlying cause of dislocation is solely overstuffed medial compartment (treated by downsizing the bearing), flexion instability (treated by upsizing the bearing), or impingement due to osteophytes (treated by removing excess bone or osteophytes) .
While bearing dislocation in mobile bearing UKA is not an uncommon complication, bearing fracture has only ever been reported twice in the literature so far [16,17,18]. The causes of bearing fracture in mobile bearing UKA include increased poly wear, use of thin poly in the initial surgery, and use of earlier design of the Oxford knee, which had higher degrees of freedom and instability of polyethylene insert [10, 19]. Lim et al. reported a case of bearing fracture in a patient 7 years after UKA with a phase III Oxford implant . The mode of failure in their case was uneven delamination of the polyethylene in the thinnest articular portion of the insert, leading to a fatigue crack that propagated . Munjal et al. reported bearing fracture in an obese patient 7 years after a phase III Oxford UKA due to sudden excessive load on a thin insert (3 mm) weakened by oxidation . Both of these cases are similar to the second case presented in our case report in that all three cases occurred late and the thinnest polyethylene insert was used in the initial surgery. Zimmer Biomet changed the shape of the polyethylene insert in their later phases of design primarily in the anterolateral portion to make it more difficult to spin and dislocate . The original phase 1 design of the Oxford partial knee had more degrees of freedom and instability of the polyethylene insert. This may have led to increased wear and delamination and subsequently to catastrophic failure in these earlier designs . Our case report and prior published studies suggest that use of a thin polyethylene insert (less than 4 mm in size) places the patient at increased risk of bearing fracture, especially with use of earlier designs of the Oxford knee.
Treatment options for catastrophic bearing failure due to fracture include bearing exchange and TKA. Revision to TKA should be considered if the bearing failure occurred due to increased contact stresses secondary to varus alignment of the knee, component malalignment, or morbid obesity of the patient . Bearing exchange can be considered if the underlying cause of failure was solely too thin a polyethylene liner. However, outcomes after bearing exchange for polyethylene wear are not well studied and long-term survivorship is unknown.