A new method of registration in navigated hip arthroplasty without the need to register the anterior pelvic plane.

A prospective clinical study of 50 patients was conducted to validate a new method of imageless computer navigated hip arthroplasty. The new method enables the surgeon to acquire all registration points with the patient positioned and draped in lateral decubitus position. The final component orientation was measured from post-operative CT scans. The mean error in component position was -1.1° (SD 3.1°) for inclination and 0.9° (SD 4.3°) for anteversion. This compared favourably with the error of -1.8° (SD 1.8°) for inclination and -4.8° (SD 2.7°) for anteversion when using the traditional APP registration. Results show that one can expect the acetabular component to be within a safe zone of ±10° in 99.8% for inclination and 97.7% for anteversion when using the new lateral registration method. Level of Evidence Level II, Prognostic study.

Anatomical relationships of human pelvises and their application to registration techniques.

Knowledge of consistent anatomical relationships is an important criterion for establishing registration procedures for orthopedic navigation systems. Based on an analysis of 420 CT data sets, we investigated whether a robust registration of the pelvis in a lateral decubitus position could be achieved based on anatomical relationships. For this purpose, we assessed basic statistics and variation in anatomical parameters. It was found that inter-teardrop and inter-fossa distances exhibit a high degree of consistency in pelvises of the same gender. Additionally, stable relationships were found between the anterior pelvic plane (APP) and other reference planes that rely on acetabular points instead of pubic points. Based on these results, a registration procedure for the pelvis was developed which uses only landmarks that are accessible intra-operatively from the ipsilateral side. The deviation between a standard APP registration and this new registration method was assessed. For a standard cup position (40° inclination, 15° anteversion), the resulting deviations were found to be 0.15 ± 2.86° for inclination and 0.27 ± 3.46° for anteversion. Of the registrations, 99% had cup positions within the Lewinnek safe zone. This shows that accurate lateral pelvis registration based on anatomical relationships is achievable.

Minimally invasive computer-navigated total hip arthroplasty, following the concept of femur first and combined anteversion: design of a blinded randomized controlled trial.

BACKGROUND: Impingement can be a serious complication after total hip arthroplasty (THA), and is one of the major causes of postoperative pain, dislocation, aseptic loosening, and implant breakage. Minimally invasive THA and computer-navigated surgery were introduced several years ago. We have developed a novel, computer-assisted operation method for THA following the concept of "femur first"/"combined anteversion", which incorporates various aspects of performing a functional optimization of the cup position, and comprehensively addresses range of motion (ROM) as well as cup containment and alignment parameters. Hence, the purpose of this study is to assess whether the artificial joint's ROM can be improved by this computer-assisted operation method. Second, the clinical and radiological outcome will be evaluated. METHODS/DESIGN: A registered patient- and observer-blinded randomized controlled trial will be conducted. Patients between the ages of 50 and 75 admitted for primary unilateral THA will be included. Patients will be randomly allocated to either receive minimally invasive computer-navigated "femur first" THA or the conventional minimally invasive THA procedure. Self-reported functional status and health-related quality of life (questionnaires) will be assessed both preoperatively and postoperatively. Perioperative complications will be registered. Radiographic evaluation will take place up to 6 weeks postoperatively with a computed tomography (CT) scan. Component position will be evaluated by an independent external institute on a 3D reconstruction of the femur/pelvis using image-processing software. Postoperative ROM will be calculated by an algorithm which automatically determines bony and prosthetic impingements. DISCUSSION: In the past, computer navigation has improved the accuracy of component positioning. So far, there are only few objective data quantifying the risks and benefits of computer navigated THA. Therefore, this study has been designed to compare minimally invasive computer-navigated "femur first" THA with a conventional technique for minimally invasive THA. The results of this trial will be presented as soon as they become available. TRIAL REGISTRATION NUMBER: DRKS00000739.

Experimental validation of a pinless femoral reference array for computer-assisted hip arthroplasty.

The use of computer navigation systems during total hip arthroplasty requires the femoral fixation of a reflective dynamic reference base (DRB), which theoretically involves the risk of bony fracture, infection, and pin loosening. The first objective of this study was to evaluate the relative movements between a novel, noninvasive external femoral DRB system and the femur. Secondly, the maximum effects of these 3D movements on intraoperative, computer-assisted leg length and offset measures were evaluated. An imageless navigation system was used to track the positions of the soft tissue attached, pinless DRB relative to an invasive reference marker on the femur during a less-invasive, anterior surgical hip approach. Relative translatory movements up to 8.2 mm mediolaterally and up to 8.8 degrees in rotation were measured. Using a measurement technique in which the calculation of leg length and offset changes is primarily based on a specific realignment of the leg, maximum differences of 1.3 mm for leg length and 1.2 mm for offset were found when comparing the pin-based and pinless methods. Thus, invasive fixation techniques with screws or pins are still the method of choice when standard measurement algorithms for intraoperative leg length and offset measures are used. Though direct translatory and rotational variations between the pinless array and the femoral bone were detected, the pinless array can be used to assess leg length and offset when used with a specific measurement technique that compensates for such variations.