F. Dorey

The femoral head/neck offset and hip resurfacing

Because the femoral head/neck junction is preserved in hip resurfacing, patients may be at greater risk of impingement, leading to abnormal wear patterns and pain. We assessed femoral head/neck offset in 63 hips undergoing metal-on-metal hip resurfacing and in 56 hips presenting with non-arthritic pain secondary to femoroacetabular impingement. Most hips undergoing resurfacing (57%; 36) had an offset ratio <or= 0.15 pre-operatively and required greater correction of offset at operation than the rest of the group. In the non-arthritic hips the mean offset ratio was 0.137 (0.04 to 0.23), with the offset ratio correlating negatively to an increasing alpha angle. An offset ratio <or= 0.15 had a 9.5-fold increased relative risk of having an alpha angle >or= 50.5 degrees. Most hips undergoing resurfacing have an abnormal femoral head/neck offset, which is best assessed in the sagittal plane.

Strength of Initial Mechanical Fixation of Screw Ring Acetabular Components

This study was conducted to determine the effects of design on the initial fixation of several types of screw-ring acetabular components. The components were tested in polyurethane foam to assess relative screw fixation strengths with a consistent material. Embalmed pelves from anatomic specimens were used to conduct paired tests between designs that showed large differences in insertional torque to failure in foam. The quality of the initial fixation in foam was found to be dependent on the design features of the components. Components with widely spaced, deep threads, and minimal thread interruptions offered the strongest initial fixation in foam. Tests in bone revealed a wide range of fixation strengths reflecting the variability in bone quality. No differences in fixation strength attributable to component design were observed in bone. When the insertional failure torque was greater than 60 N.m, one-half of the pelves fractured, and these fractures occurred with all designs. At failure torques less than 60 N.m, failure was predominantly due to thread strippage of the screw, with only two of 20 specimens experiencing pelvic fracture.

Acetabular reconstruction with a threaded prosthesis for failed total hip arthroplasty.

The MEC-Ring threaded acetabular component was used in 35 patients for revision of failed cemented hip arthroplasties, and 32 have been observed for two to four years (mean, 2.5 years). With only a short-term follow-up period, 44% of patients have already required revision for failure of the MEC-Ring acetabular component. Patients with especially poor acetabular bone stock that had had structural bone graft had a significantly higher rate of failure. The authors developed a technique of supine oblique roentgenograms that allowed better visualization of the component-bone interface. Eighty-six percent of patients had radiolucencies, and 67% of these were progressive. The presence of progressive radiolucencies, component migration, or both was associated with a higher failure rate. Progressive radiolucencies were also associated with a worse pain score in patients whose arthroplasties have not failed. The MEC-Ring threaded component is not recommended for revision hip arthroplasty, especially in cases with poor acetabular bone stock.

Porous surface replacement of the hip with chamfer cylinder design

Fifty-seven porous surface replacements (PSR) were performed in 53 patients (36 men and 17 women) with a mean age of 54 years (range, 19-75 years). Follow-up examination times ranged from one year to 2.5 years with 33 patients having follow-up periods of at least two years. Preoperative diagnoses were osteoarthritis (OA), 37; osteonecrosis (ON), six; dysplasia, nine; rheumatoid ankylosing spondylitis, three; and other, two. Sixteen hips had metal-backed acrylic-fixed THARIES (total hip articular replacement by internal eccentric shells) acetabular sockets, nine hips had a cobalt chrome hemispherical beaded acetabular component with adjuvant screws, and 32 hips had a chamfer-cylinder designed acetabulum. Pain relief has been immediate and more complete than with acrylic-fixed or biologic-in-growth stem-type replacements with comparable walking and function improvements. There have been no major systemic complications, sepsis, or loosening. There have been two transient peroneal nerve palsies and three trochanteric fibrous unions. There has been one subluxation requiring reoperation. Histologic sections of the removed femoral surface component showed excellent (90%) bone ingrowth. Circumferential progressive radiolucencies developed at the bone-cement interface by one year in all of the 16 acrylic-fixed acetabular components. Reaming or seating defects were noted in 25% of the patients on postoperative radiographs. Serial radiographic analyses demonstrate progressive narrowing of all of the chamfered cylinder design and less in hemispherical design with screw fixation. These observations are encouraging and suggest healing of the bone-component interface with bony trabeculae in the porous-coated acetabular design. This new surface replacement (SR) of the hip uses porous-ingrowth fixation to overcome the major disadvantages of acrylic-fixed SR which are as follows: (1) excessive acetabular reaming, (2) poor long-term fixation, and (3) difficulty with acetabular revision.

Tibial torque generation in a flexed weight-bearing stance.

Internal and external torque generated about the long axis of the lower extremity was measured in 18 male subjects who were instructed to twist with maximal effort against a fixed footplate containing an instrumented torque cell. Mean torque values ranged from 30 to 71 newton meters (Nm) depending upon the test conditions. Torques recorded during the flexed single-leg stance were 19% to 49% higher than those measured while seated. Values at 45 degrees of knee flexion were 11% to 16% greater than those at 20 degrees. Torques generated while wearing a ski boot were 8% to 11% greater than those recorded in an athletic shoe. When movement of the pelvis and upper torso was allowed, torque values were 17% to 49% higher than those recorded when the hips and shoulders were restrained which allowed only lower leg musculature to act in an isolated fashion. There were no differences between internal versus external generated torques when the hips and torso were restrained. When the hips and shoulders were unrestrained, internal torque was 12% greater than external torque. There were no strong correlations between generated torque and body weight or height. These generated torque values suggest that if ski bindings are set to American Society for Testing and Materials (ASTM) standards for twist-release torque, then upper torso and pelvic movement in conjunction with tensed knee musculature (i.e., a locked knee) may be necessary to accomplish binding release. Use of the lower leg musculature alone (i.e., ankle twist) may not generate sufficient torque for release.