Frank Layher

The Rationale for Tilt-Adjusted Acetabular Cup Navigation

BACKGROUND When performing total hip arthroplasty without computer navigation, surgeons align the acetabular component with landmarks such as the plane of the operating table and the presumed position of the pelvis. In contrast, first-generation computer navigation systems rely on the pelvic anterior plane, defined by the anterior superior iliac spines and the pubic tubercle. We sought to study the effect of patient positioning on the tilt of the pelvis as measured in the pelvic anterior plane and its effect on cup alignment angle values. METHODS In forty patients, the supine pelvic anterior plane tilt angle was measured with use of computed tomographic scans made before and after total hip arthroplasty (Group A). In thirty other patients undergoing total hip arthroplasty, preoperative supine pelvic anterior plane tilt angle was measured with a computed tomographic scan and the preoperative standing pelvic anterior plane tilt angle was measured on a lateral radiograph (Group B). From these data, we used hip navigation planning software to develop a nomogram providing tilt-adjusted cup angles that would align the cup in a target range of 40 degrees +/- 10 degrees of abduction and 15 degrees +/- 10 degrees of anteversion. A third group of ninety-eight patients (Group C) then underwent total hip arthroplasty with computer navigation with use of our nomogram to provide tilt-adjusted values for cup alignment. Postoperative computed tomography scans were made to evaluate cup alignment, and the patients were followed for at least one year. RESULTS In Group A, the mean preoperative supine pelvic tilt angle (and standard deviation) was -8.9 degrees +/- 6.8 degrees (forward rotation of the pelvis) and the mean postoperative angle was -10.9 degrees +/- 7.6 degrees (p < 0.05). In Group B, the mean preoperative supine pelvic tilt angle was -10.4 degrees +/- 7.4 degrees and the mean preoperative standing pelvic tilt angle was -5.0 degrees +/- 9.4 degrees (p < 0.001). In the group of ninety-eight patients who underwent navigated total hip arthroplasty (Group C), there were no dislocations at one year of follow-up. Seventy-two patients underwent postoperative computed tomography scans; 99% of cup anteversion values and 97% of cup abduction values were in the target range. CONCLUSIONS For navigation systems that rely on the pelvic anterior plane, cup alignment values can be converted to familiar target values with our nomogram with good accuracy and reproducibility. The next generation of navigation systems should be able to measure the pelvic tilt for each individual patient and automatically adjust alignment values.

Radiographic Changes in Forefoot Geometry with Weightbearing

Quality assessment of forefoot surgery depends mainly on weightbearing radiographs. A prospective study has been performed to compare the influence of weightbearing on forefoot geometry. Dorsoplantar radiographs for weightbearing and non-weightbearing conditions were performed in 99 patients. Hallux valgus angle, the intermetatarsal angles between the first and second and first and fifth metatarsals and intermetatarsal distance were measured using an interactive digitizer connected to a computer. The intermetatarsal angles showed a statistically significant increase during weightbearing. Unrelated to the severity of hallux valgus deformity, hallux valgus angles demonstrated an inverse behavior showing larger values under non-weightbearing conditions. For that reason, radiological evaluation of forefoot geometry strictly requires similar weightbearing conditions and comparable positioning of the foot.

Imageless Cup and Stem Navigation in Dysplastic Hips with the Navitrack and Vector Vision Systems

Introduction Total hip arthroplasty performed for developmental dysplasia of the hip has a higher incidence of complications than total hip arthroplasty for primary degenerative arthritis [8]. The poorer results appear to correlate with the severity of the hip deformity and thus with the misalignment of the hip prosthesis. With an incidence of between 1% and 10% in primary and in revision total hip replacements, dislocations and fractures are second in frequency only to aseptic loosening as a cause for revision surgery [9, 16]. Contrary to knee replacements, where deformities of the joint axis of only 10° are clinically apparent, mal-positions of one or both components of a stable, non-dislocated hip are difficult to recognize. Nevertheless, they are important factors influencing the incidence of impingement, polyethylene wear and aseptic loosening of hip endoprostheses [5, 10, 15]. In the past 10 years, computer-assisted surgery has been introduced for hip replacement [4]. The aim is even more precise placement of the prosthesis. The necessity of preoperative computed tomography (CT) scans in CT-based navigation has been a deterrent to practical application. For this reason, we have been involved in the development of the imageless navigation systems Navitrack (Orthosoft/Zimmer) and Vector Vision (Brainlab). These systems are intended to provide the surgeon with information not only about the anteversion and inclination of the cup, but also about the anteversion of the stem, changes in femoral offset and the resulting leg length. Technique of Imageless Navigation

Normalized gait analysis parameters are closely related to patient-reported outcome measures after total knee arthroplasty

Up till now, only a weak connection could be shown between patient-related outcome measures (PROMs) and measurements obtained by gait analysis (e.g. speed, step length, cadence, ground reaction force, joint moments and ranges of motion) after total knee arthroplasty (TKA). This may result from the methodical problem that regression analyses are performed using data that are not normalized against a healthy population. It does appear reasonable to assume that patients presenting a physiological gait pattern are content with their joint. The more the gait parameters differ from a normal gait pattern the worse the clinical outcome measured by PROMs should be expected to be. In this retrospective study, 40 patients were enrolled who had received a gait analysis after TKA, and whose PROMs had been evaluated. A gender- and age-matched control group was formed out of a group of test persons who had already undergone gait analysis. Gait analysis was undertaken using the motion analysis system 3D Vicon with ten infrared cameras and three strength measuring force plates. The physiological gait analysis parameters were deduced from arithmetic mean values taken from all control patients. The deviances of the operated patients’ gait analysis parameters from the arithmetic mean values were squared. From these values, the Pearson correlation coefficients for different PROMs were then calculated, and regression analyses were performed to elucidate the correlation between the different PROMs and gait parameters. In the regression analysis, the normalized cadence, relative gait speed of the non-operated side, and range of the relative knee moment of the operated side could be identified as factors which influence the Forgotten Joint Score (FJS-12). The explanation model showed an increase of the FJS-12 with minimisation of these normalized values corresponding to an approximation of the gait pattern seen in the healthy control group. The connection was strong, having a correlation coefficient of 0.708. A physiological gait pattern after TKA results in better PROMs, especially the FJS-12, than a non-physiological gait pattern does.

The Proximal and Distal Femoral Canal Geometry Influences Cementless Stem Anchorage and Revision Hip and Knee Implant Stability

Although cementless revision arthroplasty of the hip has become the gold standard, revision arthroplasty of the distal femur is controversial. This study evaluated the anchoring principles of different femoral revision stem designs in extended bone defect situations, taking into account the anatomical conditions of the proximal and distal femur, and the resulting primary stability. Cementless press-fit stems of 4 different designs were implanted in synthetic femurs. The specimens were analyzed by computed tomography and were tested considering axial/torsional stiffness and migration resistance. Different stem designs anchored in different femoral canal geometries achieved comparable primary stability. Despite considerably different anchorage lengths, no difference in migration behavior or stiffness was found. Both in the distal femur and in the proximal femur, the conical stems showed a combination of conical and 3-point anchorage. Regarding the cylindrical stem tested, a much shorter anchorage length was sufficient in the distal femur to achieve comparable primary stability. In the investigated osseous defect model, the stem design (conical vs cylindrical), not the geometry of the femoral canal (proximal vs distal), was decisive regarding the circumferential anchorage length. For the conical stems, it can be postulated that there are reserves available for achieving a conical-circular fixation as a result of the large contact length. For the cylindrical stems, only a small reserve for a stable anchorage can be assumed. [Orthopedics. 2018; 41(3):e369-e375.].