INTRODUCTION

Leg length discrepancy following total hip arthroplasty (THA) can be functionally disabling for affected patients and can lead on to litigation issues. Assessment of limb length discrepancy during THA using traditional methods has been shown to produce inconsistent results. The aim of our study was to compare the accuracy of navigated vs. non navigated techniques in limb length restoration in THA.

Background. Postoperative dislocation is one of the main surgical complications and the primary cause for revision surgery after 2-stage implant exchange due to periprosthetic infection of a total hip arthroplasty. Objective. The aims of our study were (1) to determine the incidence of dislocation after two-stage THA reimplantation without spacer placement, (2) to evaluate relevant risk factors for dislocation and (3) to assess the final functional outcome of those patients. Method. We prospectively analyzed 187 patients who underwent a two-stage total hip arthroplasty (THA) revision after being diagnosed with periprosthetic joint infection (PJI) from 2013 to 2019. The mean duration of follow-up was 54.2 ± 24.9 months (>36 months). The incidence of postoperative dislocation and subsequent revision was estimated through Kaplan-Meier curves and potential risk factors were identified using Cox hazard regression. The functional outcome of the patients was assessed using the modified Harris Hip Score (mHHS). Results. The estimated cumulative dislocation-free survival was 87.2% (95% CI: 81.2%-91.3%) with an estimated 10% and 12% risk for dislocation within the first 6 and 12 months, respectively. The use of a dual-mobility construct had no significant impact on the dislocation rate. Increasing body mass index (BMI) (HR=1.11, 95% CI: 1.02-1.19, p=0.011), abductor mechanism impairment (HR=2.85, 95% CI: 1.01-8.01, p=0.047), the extent of elongation of the affected extremity between stages (HR=1.04, 95% CI: 1.01-1.07, p=0.017), the final leg length discrepancy (HR=1.04, 95% CI: 1.01-1.08, p=0.018) and PJI recurrence (HR=2.76, 95% CI: 1.00-7.62, p=0.049) were found to be significant risk factors for dislocation. Overall revision rates were 17% after THA reimplantation. Dislocated hips were 62% more likely to undergo re-revision surgery (p<0.001, Log-rank= 78.05). A significant average increase of 30 points in mHHS scores after second-stage reimplantation (p=0.001, Wilcoxon-rank) was recorded, but no difference was noted in the final HHS measurements between stable and dislocated hips. Conclusion. Dislocation rates after 2-stage THA reimplantation for PJI remain high, especially regarding overweight or re-infected patients. Careful leg length restoration and an intact abductor mechanism seem critical to ensure stability in these complex patients

Introduction. Correct postoperative leg length restoration is among the most important goals of hip arthroplasty. Therefore, we developed, validated and clinically applied a novel software algorithm based on surgical navigation, which allows the surgeon to restore a defined femur position without establishing a femoral coordinate system or the hip joint center and measure the leg length accurately and simply. Material and Methods. This new leg length algorithm was used in 154 hips (145 patients) that underwent CT-based computer-assisted THA (VectorVision Build 274 prototype; BrainLAB AG, Helmstetten, Germany) with a tissue preserving superior capsulotomy. Intraoperatively, a pelvic and a femoral dynamic reference bases (DRB) were applied and the anterior pelvic plane (APP) was set as the pelvic coordinate system. Then, the hip joint was put in a neutral position and this position, and the relative position of the femoral DRB relative to the pelvic DRB, was captured and stored by the navigation system. After implantation of the prosthesis the same above described femoral position with the same amplitude of flexion/extension, abduction/adduction and rotation was restored. Now, any resulting difference was due to linear changes. Validation of this new algorithm was performed by comparing the navigated results to measurements from calibrated antero-posterior pre- and postoperative radiographs. The radiographic results were compared to the mean leg length change measured with the navigation system. Results. No significant difference was found between radiographic leg length change and the results from the navigation system (p=0.658). The mean difference between the radiographic results and the results from the navigation system was −0.5 (1–8 mm (range, −5–4 mm). The mean registration accuracy of the navigation system was 2.04 (0.58 mm (range, 0.70–3.00 mm). Discussion. This novel tool has the potential to increase the accuracy and consistency of leg-length change measurement during hip arthroplasty. Improved methods of measuring leg length change during surgery are even more critical now, when smaller incisions are being used, because traditional mechanical measurement methods are potentially even more unreliable than they are when larger exposures are used. This current method of measuring leg length change eliminates the need to calculate the center of rotation of the arthritic hip joint, which is often not accurately possible, and eliminates the need to establish a femoral coordinate system, which can be time consuming and frustrating. Besides registration accuracy, validation with plain radiographs is another potential source of error. Nonetheless, there was a substantial agreement between the radiographic results and the results from the navigation system. This novel computer-assisted method represents an accurate and simple tool for intraoperative leg length measurement

The number one reason to consider large heads in total hip arthroplasty (THA) is for increased stability. Large diameter femoral heads substantially increase stability by virtue of increased range of motion and increased jump distance, which is the amount of displacement required to sublux the head out of the socket. Prevention is the best means for reducing dislocation, with requisites for stability being appropriate component position, restoration of leg length, and restoration of offset. In a review from our center studying the frequency of dislocation with small diameter femoral heads (≤32 mm) in 1262 patients (1518 hips) who underwent primary THA performed via a direct lateral approach, we observed a dislocation rate of 0.8% (12 of 1518). In a subsequent study of 1748 patients (2020 hips) who underwent primary THA at our center with large diameter heads (mean 43 mm, range 36–60 mm), we observed a substantially lower 0.04% frequency of dislocation (one of 2010) at a mean followup of 2.6 years. Our findings have been echoed in studies from several other centers. Howie et al. reported a prospective controlled trial of 644 low risk patients undergoing primary or revision THA randomised to receive either a 36 mm or 28 mm metal head articulated on highly crosslinked polyethylene. They observed significantly lower frequency of frequency of dislocation with 36 mm heads both overall (1.3%, 4 of 299 versus 5.4%, 17 of 216 with 28 mm heads, p=0.012) and in primary use (0.8%, 2 of 258 versus 4.4%, 12 of 275 with 28 mm heads, p=0.024), and a similar trend in their smaller groups of revision patients (5%, 2 of 41, versus 12%, 5 of 41 with 28 mm heads, p=0.273). Lachiewicz and Soileau reported on early and late dislocation with 36- and 40 mm heads in 112 patients (122 hips) at presumed high risk for dislocation who underwent primary THA. Risk factors were age >75 for 80 hips, proximal femur fracture for 18, history of contralateral dislocation for 2, history of alcohol abuse in 2, large acetabulum (>60 mm) in 6, and other reasons in 14. Early dislocation (<1 year) occurred in 4% (5 of 122), all with 36 mm heads. Late dislocation (>5 years) did not occur in any of the 74 patients with followup beyond 5 years. Stroh et al. compared 225 patients (248 hips) treated with THA using small diameter heads (<36 mm) to 501 patients (559 hips) treated with THA using large diameter heads (≥36 mm). There were no dislocations with large diameter heads compared with 1.8% (10 of 559) with small diameter heads. Allen et al. studied whether or not large femoral heads improve functional outcome after primary THA via the posterior approach in 726 patients. There were 399 done with small heads (<36 mm), 254 with medium heads (36 mm), and 73 with large heads (>36 mm), analyzed pre-operatively, at 6 months, and at 12 months. The authors could not find a correlation between increasing head size and improved function at one year, but observed that dislocation was reduced with large diameter heads. Optimization of hip biomechanics via proper surgical technique, component position, and restoration of leg length and offset are mandatory in total hip arthroplasty. Large heads enhance stability by increasing range of motion prior to impingement and enhancing jump stability

The number one reason to consider large heads in total hip arthroplasty (THA) is for increased stability. Large diameter femoral heads substantially increase stability by virtue of increased range of motion and increased jump distance, which is the amount of displacement required to sublux the head out of the socket. Prevention is the best means for reducing dislocation, with requisites for stability being appropriate component position, restoration of leg length, and restoration of offset. In a review from our center studying the frequency of dislocation with small diameter femoral heads (≤32 mm) in 1262 patients (1518 hips) who underwent primary THA performed via a direct lateral approach, we observed a dislocation rate of 0.8% (12 of 1518). In a subsequent study of 1748 patients (2020 hips) who underwent primary THA at our center with large diameter heads (mean 43 mm, range 36–60 mm), we observed a substantially lower 0.04% frequency of dislocation (one of 2010) at a mean followup of 2.6 years. Our findings have been echoed in studies from several other centers. Howie et al. reported a prospective controlled trial of 644 low risk patients undergoing primary or revision THA randomised to receive either a 36 mm or 28 mm metal head articulated on highly crosslinked polyethylene. They observed significantly lower frequency of frequency of dislocation with 36 mm heads both overall (1.3%, 4 of 299 versus 5.4%, 17 of 216 with 28 mm heads, p=0.012) and in primary use (0.8%, 2 of 258 versus 4.4%, 12 of 275 with 28 mm heads, p=0.024), and a similar trend in their smaller groups of revision patients (5%, 2 of 41 versus 12%, 5 of 41 with 28 mm heads, p=0.273). Lachiewicz and Soileau reported on early and late dislocation with 36- and 40 mm heads in 112 patients (122 hips) at presumed high risk for dislocation who underwent primary THA. Risk factors were age >75 for 80 hips, proximal femur fracture for 18, history of contralateral dislocation for 2, history of alcohol abuse in 2, large acetabulum (>60 mm) in 6, and other reasons in 14. Early dislocation (<1 year) occurred in 4% (5 of 122), all with 36 mm heads. Late dislocation (>5 years) did not occur in any of the 74 patients with follow up beyond 5 years. Stroh et al. compared 225 patients (248 hips) treated with THA using small diameter heads (<36 mm) to 501 patients (559 hips) treated with THA using large diameter heads (≥36 mm). There were no dislocations with large diameter heads compared with 1.8% (10 of 559) with small diameter heads. Allen et al. studied whether or not large femoral heads improve functional outcome after primary THA via the posterior approach in 726 patients. There were 399 done with small heads (<36 mm), 254 with medium heads (36 mm), and 73 with large heads (>36 mm), analyzed preoperatively, at 6 months, and at 12 months. The authors could not find a correlation between increasing head size and improved function at one year, but observed that dislocation was reduced with large diameter heads. Optimization of hip biomechanics via proper surgical technique, component position, and restoration of leg length and offset are mandatory in total hip arthroplasty. Large heads enhance stability by increasing range of motion prior to impingement and enhancing jump stability