Undersizing of the tibial component in Oxford unicompartmental knee arthroplasty (UKA) increases the risk of periprosthetic fractures.

INTRODUCTION: Tibial periprosthetic fractures (TPF) after unicompartmental knee arthroplasty (UKA) are a rare condition that affects about 1% of cases. Known risk factors include age, sex, body mass index (BMI), and bone density, as well as surgical technique and prosthesis design. The purpose of the study was to determine if undersizing of the tibial component in relation to the femoral component increases the risk of tibial periprosthetic fractures. MATERIAL AND METHODS: Over a 6-year-period 1542 patients with cemented (n = 363) and uncemented (n = 1179) medial UKA were retrospectively evaluated. Tibial periprosthetic fractures were identified and classified, and epidemiologic data were documented at follow-up. Undersizing was defined as a smaller tibial component compared to the femoral implant. The association of potential risk factors for TPF with the incidence of TPF was investigated with binominal logistic regression. RESULTS: Fourteen patients (0.9%) suffered from TPF at a median of 1 month after surgery. The mean follow-up period was 5.9 ± 1.7 years. Fractures were more common in cases with undersized tibial components [odds ratio (OR) 3.2, p < 0.05]. Furthermore, older age (OR 1.1, p < 0.05) and female sex (OR 6.5, p < 0.05) were identified as significant risk factors, while BMI (p = 0.8) and cemented implantation (p = 0.2) had no effect on fracture rate. Revision surgery included open reduction and internal fixation or conversion to total knee arthroplasty. CONCLUSIONS: Undersizing of implant sizes in UKA increases the risk for TPF especially in patients with small tibial implants. Therefore, mismatched implants should be avoided for UKA particularly when risk factors like obesity, older age, or female gender are present. Tibial periprosthetic fractures were successfully treated by open reduction and internal fixation or conversion to total knee arthroplasty.

Impact of keel saw blade design and thickness on the incidence of tibial plateau fracture and tibial implant-loosening in cementless medial UKR.

BACKGROUND: Tibial plateau fractures and tibial implant- loosening are severe complications in cementless unicompartmental knee replacement (UKR). The tibial keel preparation is particularly demanding and different saw blades can be used. It was hypothesized that different blade designs and thickness have an influence on the frequency of tibial plateau fractures and implant-loosening in cementless medial UKR. METHODS: 1258 patients with cementless medial UKR were included in this retrospective study between 2013 and 2020. The tibial keel cut was performed either with a double keel saw blade (DKS; 2.8 mm) and added hand guided pick or a mono reciprocating saw blade (RKB) of different thickness (2.5 mm; 2.65 mm; 2.75 mm). Tibial plateau fracture and loosening were demonstrated by standard two-plane radiographs. Tibial implant-loosening was defined as complete radiolucency and implant migration. Fracture and loosening were combined with pain and loss of function. RESULTS: In 126 patients (10%) the tibial keel was prepared with DKS, in 407 patients (32.4%) with RKB 2.5 mm, in 330 patients (26.2%) with RKB 2.65 mm and in 395 patients (31.4%) with 2.75 mm. In 4 patients (3.17%) with DKS tibial plateau fracture occurred, in 4 patients (0.99%) with 2.5 mm RKB, in 3 patients (0.92%) with 2.65 mm RKB and in 1 patient (0.25%) with 2.75 mm RKB. Significantly fewer fractures occurred with a RKB design (p = 0.007). A negative correlation between fracture incidence and RKB saw blade thickness was found (Spearman-r = - 0.93). No difference for tibial implant-loosening was shown (p = 0.51). CONCLUSION: Different blade designs and thickness have a significant influence on the incidence of tibial plateau fractures and aseptic tibial implant-loosening. The incidence of tibial plateau fractures in cementless medial UKR can be reduced by changing the design and thickness of the tibial keel saw blade. Greater thickness of RKB leads to significantly fewer tibial plateau fractures while the incidence of implant-loosening is not increasing. TRIAL REGISTRATION: This study was retrospectively registered and ethical approval was waived by the local ethical committee (No. 2020-1174).

Implantation Accuracy of a Lateral Unicompartmental Knee Arthroplasty: A Hoffa´s Fat Pad-Preserving Medial Approach Versus the Transpatellar Lateral Approach.

BACKGROUND: A small proportion of patients suffer from isolated lateral osteoarthritis where the sole lateral unicompartmental knee arthroplasty (UKA) is a possible treatment option. There, both a medial and a lateral surgical approach can be considered. This study should answer the question whether the lateral approach is superior to a modified medial approach in terms of implantation accuracy and subjective outcome. METHODS: In this retrospective study, 175 patients with lateral UKA were included between 2015 and 2020. In 82 patients, the lateral approach was used, and in 93 patients, the medial approach was used. To assess implantation accuracy, different imaging criteria on postoperative radiographs were analyzed. Postoperative patient-related outcome measurements (PROMs) (OKS, LEFS, and EQ5D) were evaluated. Statistical significance was assumed for P < .05. RESULTS: The tibial implant relation to the tibial plateau diameter in the lateral approach was significantly larger than in the medial approach (23.6% vs 22.2%; P < .001). Significantly more deviations >15° regarding flexion position of the femoral implant (P = .002) and a higher number of deviations of the slope was found (P = .06) in the lateral approach. The lateral approach showed a significantly higher rate of lateral positioning of the femoral component (P = .007). Post-PROMs showed significant improvement in both approaches. CONCLUSION: The lateral approach is not superior regarding different radiological accuracy criteria. The Hoffa´s fat pad-preserving medial approach showed good results in implantation accuracy and therefore is a good alternative to implant lateral UKA. In addition, significant improvement in PROMs could be demonstrated.

The influence of patellar resurfacing on patellar kinetics and retropatellar contact characteristics.

BACKGROUND: Femoropatellar complications are one of the most common problems after total knee arthroplasty (TKA). However, the question of whether to resurface the patella remains controversial. Therefore, we evaluated the kinetics and the retropatellar contact characteristics of patella resurfacing with fixed and gliding surfaces. METHODS: Eight Thiel-embalmed cadaver knees were tested--first intact, then after TKA without patellar resurfacing, and finally with additional patellar resurfacing--while flexing the knee from 0° to 100°. We tested a fixed as well as a gliding patella surface. During the examination, quadriceps and hamstring forces were applied. The retropatellar pressure was determined with a special patella sensor, and the patellar kinetics were measured using an optical three-dimensional motion analysis system. RESULTS: Resurfacing the patella caused a significant increase in retropatellar pressure and a significant decrease in retropatellar contact area. Using a fixed patella, the retropatellar pressure nearly quadrupled in higher flexion compared to the native patella. Furthermore, the lateral movement of the patella increased after TKA, especially after additional patellar resurfacing. CONCLUSIONS: Resurfacing the patella routinely is not advised. When osteoarthritis of the patella is found, the gliding patella should be preferred.

Influence of total knee arthroplasty on patellar kinematics and contact characteristics.

PURPOSE: Femoro-patellar complications are one of the most common problems after total knee arthroplasty (TKA). TKA components that reduce patellar loads and preserve physiological patellar kinematics should reduce these problems. Therefore, we evaluated the patellar kinematics and the retro-patellar contact characteristics in both the intact knee and in the TKA-knee. METHODS: Eight Thiel-embalmed cadaver knees were tested first intact and then after TKA using rotating as well as gliding inlay and with additional patellar resurfacing while flexing the knee from 0° to 100°. During the examination quadriceps and hamstring forces were applied. RESULTS: TKA with additional patellar resurfacing led to an increased retro-patellar pressure, a decreased contact area and an increased lateral movement. Although patellar kinematics could not be changed by using a gliding inlay compared to a rotating inlay, the gliding inlay improved retro-patellar contact characteristics by reducing the pressure and increasing the contact area, especially in higher flexion. CONCLUSIONS: The increased retro-patellar pressure together with the increased lateral movement of the patella after TKA may be one important cause for anterior knee pain appearing after TKA. In view of the improved retro-patellar contact characteristics using a gliding inlay this inlay should be preferred, providing that the posterior cruciate ligament is intact.