Martin Haimerl

The association between Femoral Tilt and impingement-free range-of-motion in total hip arthroplasty

BackgroundThere is a complex interaction among acetabular component position and antetorsion of the femoral stem in determining the maximum, impingement-free prosthetic range-of-motion (ROM) in total hip arthroplasty (THA). By insertion into the femoral canal, stems of any geometry follow the natural anterior bow of the proximal femur, creating a sagittal Femoral Tilt (FT). We sought to study the incidence of FT as measured on postoperative computed tomography scans and its influence on impingement-free ROM in THA.MethodsThe incidence of the postoperative FT was evaluated on 40 computed tomography scans after cementless THA. With the help of a three-dimensional computer model of the hip, we then systematically analyzed the effects of FT on femoral antetorsion and its influence on calculations for a ROM maximized and impingement-free compliant stem/cup orientation.ResultsThe mean postoperative FT on CT scans was 5.7° ± 1.8°. In all tests, FT significantly influenced the antetorsion values. Re-calculating the compliant component positions according to the concept of combined anteversion with and without the influence of FT revealed that the zone of compliance could differ by more than 200%. For a 7° change in FT, the impingement-free cup position differed by 4° for inclination when the same antetorsion was used.ConclusionsA range-of-motion optimized cup position in THA cannot be calculated based on antetorsion values alone. The FT has a significant impact on recommended cup positions within the concept of “femur first” or “combined anteversion”. Ignoring FT may pose an increased risk of impingement as well as dislocation.

Development and evaluation of an image-free computer-assisted impingement detection technique for total hip arthroplasty.

Periprosthetic or bony impingement in total hip arthroplasty (THA) has been correlated to dislocation, increased wear, reduced postoperative functionality with pain and/or decreased range of motion (ROM). We sought to study the accuracy and assess the reliability of measuring bony and periprosthetic impingement on a virtual bone model prior to the implantation of the acetabular cup with the help of image-free navigation technology in an experimental cadaver study. Impingement-free ROM measurements were recorded during minimally invasive, computer-assisted THA on 14 hips of 7 cadaveric donors. Preoperatively and postoperatively the donors were scanned using computed tomography (CT). Impingement-free ROM on three-dimensional CT-based models was then compared with corresponding, intraoperative navigation models. Bony/periprosthetic impingement can be detected with a mean accuracy limit of below 5° for motion angles, which should be reached after THA for activities of daily living with the help of image-free navigation technology.

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.

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° 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.

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.