[Importance of the tibial slope in knee arthroplasty]. / Bedeutung des tibialen Slopes in der Knieendoprothetik.

Notwithstanding the contributions of soft tissue restraints on postoperative kinematics and long-term survival after total knee arthroplasty (TKA), there is an emerging consensus that the underlying anatomy, especially the posterior inclination of the tibial plateau in the sagittal plane (tibial slope), might just have a comparable impact. However, this has not been fully elucidated as yet. Therefore, a thorough literature search, analysis and presentation of current scientific data was conducted. The tibial slope has been shown to relate linearly to the postoperative range of motion and function of the extensor mechanism. Furthermore, it impacts wear of the tibial insert and loosening, as well as instability of the TKA. As no consensus has been reached on the ideal tibial slope, recommendations range from 0° to 10°. Notably, more recent studies favor reconstructing the native, preoperative tibial slope, and the majority of authors advocate that knowledge of this is crucial for optimal TKA surgery.

Assessment of the tibial slope is highly dependent on the type and accuracy of the preceding acquisition.

BACKGROUND: Precise measurement of the tibial slope (TS) is crucial for realignment surgery, ligament reconstruction, and arthroplasty. However, there is little consensus on the ideal assessment. It was hypothesized that the tibial slope changes according to the acquisition technique and both tibial length as well as femoral rotation serve as potential confounders. METHODS: 104 patients (37 women, 67 men; range 12-66 years) were retrospectively selected, of which all patients underwent a 1.5-Tesla MRI and either additional standard lateral radiographs (SLR, n = 52) or posterior stress radiographs (PSR, n = 52) of the index knee. Two blinded observers evaluated the medial tibial slope as the medial TS is primarily used in clinical practice. Additionally, the length of the diaphyseal axis and the extent of radiographic malrotation were measured. RESULTS: Mean TS on MRI was significantly lower compared to radiographs (4.2° ± 2.9° vs. 9.1° ± 3.6°; p < 0.0001). There was a significant correlation between MRI and PSR (p < 0.0001 with r = 0.7), but not with SLR (p = 0.93 with r = 0.24). Tibial length was a significant predictor for the difference between MRI and SLR (regression coefficient ß = - 0.03; p = 0.035), yet not between MRI and PSR (ß = - 0.003; p = 0.9). Femoral rotation proved to be a significant predictor for the agreement between both observers (PSR: ß = 0.14; p = 0.001 and SLR: ß = 0.08; p = 0.04). ICC indicated a high interrater agreement for the radiographic assessment (ICC ≥ 0.72). CONCLUSIONS: There is a substantial variance between MRI and radiographic measurement of the tibial slope. However, as MRI assessment is time-consuming and requires specialized software, instrumented radiographs might be an alternative. Due care has to be taken to ensure that radiographs contain a sufficient tibial length, and femoral rotation is avoided. STUDY DESIGN: Case series (diagnosis); Level of evidence, 4.

Influence of age and mechanical stability on bone defect healing: age reverses mechanical effects.

Non-unions and delayed healing are still prevalent complications in fracture and bone defect healing. Both mechanical stability and age are known to influence this process. However, it remains unclear which factor dominates and how they interact. Within this study, we sought a link between both factors. In 36 female Sprague-Dawley rats, the left femur was osteotomized, distracted to an osteotomy gap of 1.5 mm and externally fixated. Variation of age (12 vs. 52 weeks - biologically challenging) and fixator stiffness (mechanically challenging) resulted in 4 groups (each 9 animals): YS: young semi-rigid, OS: old semi-rigid, YR: young rigid and OR: old rigid. Qualitative and quantitative radiographical analyses were performed at weeks 2, 4 and 6 after surgery. Six weeks post-op, rats were sacrificed and femora were harvested for biomechanical testing (torsional stiffness (TS) and maximum torque at failure (MTF)). Six weeks after surgery, TS showed a significant interaction between age and fixation stiffness (p<0.0001). TS in YR was significantly higher than that in the other groups (YS: p<0.001; OR: p<0.001; OS: p<0.001). Additionally, YS showed a significantly higher TS compared to the OS (p=0.006) and OR (p=0.046). Testing of MTF showed a significant interaction of both variables (p=0.0002) and led to significant differences between OR and YS (p<0.001), OS (p=0.046) and YR (p<0.001). The YR showed a higher MTF compared to YS (p=0.012) and OS (p=0.001), whereas OR's MTF was inferior compared to OS. At 2-week follow-up, YR (p=0.006), and at 6-week follow-up, YS and YR (p=0.032) showed significantly higher radiographic scores. At 2-week follow-up, YS's callus was larger than that of the old groups (OS: p=0.025; OR: p=0.003). In YR a significantly smaller callus was observed compared to YS at time points 4 and 6 weeks (p=0.002 for both) and compared to OS at 6-week follow-up (p=0.03). The effect of age seems to invert the effect of mechanical properties of the callus, which was not correlated to callus size. Optimization of mechanics alone seems to be not sufficient. The underlying mechanisms and causes of the age-related influences and their clinical counterparts need to be further investigated.