Evidence of trochlear dysplasia in femoral component designs.

PURPOSE: The study aimed to compare trochlear profiles of various total knee arthroplasty (TKA) models to anatomic profiles observed in healthy and pathologic knees and to evaluate trochlear designs against radiologic indicators for PF disorders and trochlear dysplasia. METHODS: The trochlear profiles of 14 different TKA models were digitized using a coordinate measurement machine at various flexion angles (0°, 15°, 30° and 45°) to deduce the following variables: sulcus angle, trochlear groove orientation, height of lateral facet, and mediolateral groove position. The effect of externally rotating the femoral component on those variables was simulated. RESULTS: The sulcus angle was greater than the indicators for trochlear dysplasia of 144° in 11 implants at 45° flexion, and in 13 implants at 30° flexion. The lateral facet height was less than average anatomic values of 5 mm in eight specimens through the entire range of early flexion (0°-30°). The trochlear groove was oriented laterally in 13 specimens (3.3°-11.7°) and was vertical in one specimen (0.3°). Applying an external rotation up to 6° resulted in noticeable lateral translation of the trochlear groove and facets, but negligible posterior translation. CONCLUSIONS: The study presented a detailed description of previously overlooked TKA design parameters and revealed that some femoral components exhibit characteristics of trochlear dysplasia. The clinical relevance of this descriptive study is that surgeons should be aware of such design limitations to improve choice of implant for patients with history of PF disorders and to adapt surgical techniques as necessary to optimize PF tracking. LEVEL OF EVIDENCE: Case-control study, Level III.

Is the anterior tibial tuberosity a reliable rotational landmark for the tibial component in total knee arthroplasty?

To analyze the morphology of the tibial plateau, we studied 100 computed tomographic scans of arthritic knees and measured the mediolateral (ML) and anteroposterior (AP) dimensions as well as their aspect ratio using 3 reference axes of rotation: transepicondylar axis (TEA), posterior tibial margin (PTM), and anterior tibial tuberosity (ATT) axis. Relative to the TEA, the PTM was internally rotated by 1.6° ± 5.1°, and the ATT externally rotated by 14.8° ± 7.2°. The AP and ML dimensions and aspect ratio differ significantly when the reference axis was ATT compared with PTM or TEA and variations were greater while using ATT axis. Our data demonstrate (1) that design of the tibial component restricts the choice of rotational alignment and (2) that ATT is not a reliable landmark for rotation of the tibial component.

Measurement of femoral head penetration in polyethylene using a 3-dimensional CT-scan technique.

BACKGROUND: Current techniques for measuring in vivo polyethylene wear suffer from a range of problems, resulting in an unacceptable lack of repeatability and/or insufficient accuracy when they are used to measure the low wear rates associated with new, highly crosslinked polyethylene. We describe an improved CT method for measurement of 3D femoral head penetration in PE acetabular cups that has sufficient accuracy and repeatability to allow assessment of the wear potential of modern implants. METHOD: The accuracy and repeatability of the CT-scan method was determined by blindly repeating measurements on a precisely calibrated 28-mm prosthetic head and by comparing them with direct metrological measurements on 10 acetabular specimens with in vitro wear from machining, and on 8 explanted acetabular specimens with in vivo wear. RESULTS: The intra- and interobserver errors in femoral head diameter were 0.036 mm (SD 0.044) and 0.050 mm (SD 0.022), respectively. CT estimated femoral head penetration in both all-poly and metal-backed acetabular components with accuracy ranging from 0.009 to 0.245 mm (mean 0.080; SD 0.067). INTERPRETATION: We found that the CT method is rapid, is accurate, and has repeatability and ease of availability. Using a slice thickness of 0.0625 mm, this method can detect wear­and also the threshold for the wear rate that causes osteolysis­much earlier than previous methods.

The asymmetric profile of the acetabulum.

Despite the curvaceous profile of the acetabulum, orthopaedic surgeons have continued to implant hemispheric cups since the introduction of total hip arthroplasty. The geometric discrepancies between the natural acetabulum and implant can result in painful iliopsoas impingement attributable to prosthetic overlap at the anterior acetabular ridge over which the iliopsoas tendon extends to leave the pelvis. We expanded on previous in vitro observations of acetabular morphology using a large in vivo sample and quantified the dimensions of the psoas valley. We studied computed tomographic scans of 200 healthy hips from 50 men and 50 women. The acetabular ridges were digitized on three-dimensional bone reconstructions and their coordinates were manipulated in spreadsheets to deduce acetabular diameter, anteversion, and inclination and to plot the rim profile. Our results confirm the acetabular rim is an asymmetric succession of three peaks and three troughs. The psoas valley has the following shape distribution: 79% curved, 11% angular, 10% irregular, and 0% straight. The mean depth of the psoas valley is 5 mm and the latitude of its trough is on average 6 mm below the acetabular equator. The use of side-specific cups that replicate the curvaceous acetabular profile could prevent prosthetic overlap and reduce the incidence of iliopsoas impingement.

Hemispheric cups do not reproduce acetabular rim morphology.

BACKGROUND: Iliopsoas impingement is a recurrent complication following THA, caused by muscle friction against a protrusive prosthetic cup.This study was designed to quantify the dimensional variations in acetabular rim profiles, with particular regard to the iliopubic valley, in order to suggest means to prevent iliopsoas impingement. MATERIAL AND METHODS: 34 cadaver pelvises were analyzed using a hip navigation system. The morphometric data were processed to plot profiles of all acetabular rims with particular regard to the shape and depth of the psoas valley. RESULTS: The acetabular rim is an asymmetric succession of 3 peaks and 3 troughs. The psoas valley is a salient feature in most pelvises and there is only a weak correlation between its depth (mean 3.8 mm, SD 2.0) and acetabular diameter, anteversion, or inclination. INTERPRETATION: It would be difficult to obviate the anterior overlap of the acetabulum using a hemispheric cup, a fortiori in certain morphotypes, without compromising range of motion or risk of dislocation. The solution for prevention of iliopsoas impingement would be to adapt cup design to acetabular anatomy, which may require different implants for the right and left sides, and hence a doubled inventory.