T. A. Gruen

Factors related to failure of structural bone grafts in acetabular reconstruction of total hip arthroplasty

Structural bone grafts in acetabular reconstruction of total hip arthroplasty have come into more common usage with the increasing number of revision arthroplasties. The authors present the outcome of 40 bone grafts with follow-up periods of 2-8 years. Primary and revision arthroplasties with both cemented and noncemented fixation are included. Factors that had a statistically significant relationship to failure of the bone graft included fit and fixation of bone graft to host, fit and fixation of prosthesis to host, union of bone graft to host, and the presence or absence of the confluence of the anterior and posterior columns of the acetabulum. Other findings of interest were a higher failure rate of multiple grafts as compared to a single graft and freeze-dried femoral heads. Migration of the socket of more than 3 mm is statistically related to failure of the socket.

Midterm Results with the PFC Sigma Total Knee Arthroplasty System

We prospectively studied 207 consecutive patients (284 knees) undergoing total knee arthroplasty (June 1996 to December 1997) with a cemented tricompartmental Sigma PFC (DePuy Orthopaedics, Warsaw, Ind) total knee arthroplasty via a standard procedure (median follow-up, 87 months). Cruciate-retaining (272 knees, 96%) and cruciate-substituting (12, 4%) implants were used. There was one revision secondary to a ligament disruption after a fall. No implants were radiographically loose or at risk for loosening. Radiolucencies (none>2 mm or progressive) were shown on anteroposterior (7% of medial tibias) and lateral (17% of posterior femora) radiographs. Knee Society pain scores improved significantly (preoperative median, 20 points; postoperative median, 50 points [P<.001]). The PFC Sigma Knee system has excellent midterm durability.

Techniques for preoperative planning and postoperative evaluation of noncemented hip arthroplasty

A technique is standardized from which the funnel shape of the intramedullary canal and quality of cortical bone of the proximal femur can be measured on prcopenuive radiographs. The same points of measurement are used to calculate the percentage fill of noncemented femoral components. A cadaveric study was undertaken to test the validity of this radiographic technique Bone stock was qualitatively assessed from 100 radiographs as type A, B, or C and quantitatively by calculating the cortical index, which is the determination of the relative thickness of cortical bone. Type A bone had an average calcar-to-canal isthmus (CC) ratio of .56 with a cortical index of .59 on the anteroposterior (AP) and .53 on the lateral radiographs. Type B bone had an average CC ratio of .39 with a cortical index of .51 on the AP and .4-1 on the lateral radiographs. Type C bone had a CC ratio of .65 with a cortical index of A1 on the AP and .29 on the lateral radiographs. The lateral radiograph may yield information critical to the clinician in preopcrative planning. The stem-bone ratio statistically correlated with bone type and was .-13 on the AP and AI on the lateral for type A bone, .-19 on the AP and A4 on the lateral for type B bone, and .53 on the AP and AS on the lateral radiographs for type C bone. By using a simple standardized technique, the surgeon will be able to predict the outcome of noncemented femoral components and provide a common ground for comparing the results of noncemented hip arthroplasties.

Cortical Bone Viscoelasticity and Fixation Strength of Press-Fit Femoral Stems: An In-Vitro Model

Cementless total hip femoral components rely on press-fit for initial stability and bone healing and remodeling for secondary fixation. However, the determinants of satisfactory press-fit are not well understood. In previous studies, human cortical bone loaded circumferentially to simulate press-fit exhibited viscoelastic, or time dependent, behavior. The effect of bone viscoelastic behavior on the initial stability of press-fit stems is not known. Therefore, in the current study, push-out loads of cylindrical stems press-fit into reamed cadaver diaphyseal femoral specimens were measured immediately after assembly and 24 h with stem-bone diametral interference and stem surface treatment as independent variables. It was hypothesized that stem-bone interference would result in a viscoelastic response of bone that would decrease push-out load thereby impairing initial press-fit stability. Results showed that push-out load significantly decreased over a 24 h period due to bone viscoelasticity. It was also found that high and low push-out loads occurred at relatively small amounts of stem-bone interference, but a relationship between stem-bone interference and push-out load could not be determined due to variability among specimens. On the basis of this model, it was concluded that press-fit fixation can occur at relatively low levels of diametral interference and that stem-bone interference elicits viscoelastic response that reduces stem stability over time. From a clinical perspective, these results suggest that there could be large variations in initial press-fit fixation among patients.

Time-Dependent Circumferential Deformation of Cortical Bone Upon Internal Radial Loading

Short and long duration tests were conducted on hollow femoral bone cylinders to study the circumferential (hoop) creep response of cortical bone subjected to an intramedullary radial load. It was hypothesized that there is a stress threshold above which nonlinear creep effects dominate the mechanical response and below which the response is primarily determined by linear viscoelastic material properties. The results indicate that a hoop stress threshold exists for cortical bone, where creep strain, creep strain rate and residual strain exhibited linear behavior at low hoop stress and nonlinear behavior above the hoop stress threshold. A power-law relationship was used to describe creep strain as a function of hoop stress and time and damage morphology was assessed.

In Vivo Performance of Moderately Crosslinked, Thermally Treated Polyethylene in a Prospective Randomized Controlled Primary Total Knee Arthroplasty Trial

Cross-linked bearings have been developed for use in total knee arthroplasty that exhibit improved wear properties, but at the expense of a decrease in mechanical strength of the cross-linked material. Adoption has been slow due to fears of mechanical failure secondary to this alteration in mechanical properties. This prospective, randomized study compared mid-term survivorship, clinical and radiographic results of a conventional polyethylene (GVF) to a cross-linked polyethylene (XLK) in total knee prostheses of the same design. At minimum 5-year follow-up there was no difference in survivorship, clinical performance or radiographic findings between the groups. There were no revisions for polyethylene wear, osteolysis or tibial insert dissociation. Most importantly, there were no revisions for mechanical failure or fracture of the polyethylene bearing in either group.

The Relationships Between Femoral Cortex Geometry and Tissue Mechanical Properties

Bone tissue and geometry are constantly modified through modeling and remodeling at the periosteal, endosteal and intracortical envelopes. Results from several studies indicate that femoral bone geometry is a predictor of whole bone strength (e.g. femoral neck strength), however, it is not known whether there is a relationship between bone structural and material properties. Bone geometry can be determined from parameters based on plane X-ray radiogrammetry which are used to evaluate femoral bone quality for implant success. If there is a relationship between these parameters and tissue mechanical properties, this would have implications in the interpretation of such parameters for assessment of fracture risk and in further understanding of bone biology. Following measurement of radiogrammetric parameters from antero-posterior and medio-lateral X-rays (cortical thickness, bone diameter, bone area, moment of inertia, cortical index, Singh index), human femurs were machined into standard test specimens for assessment of tensile fracture toughness (GIc) of the tissue. Results indicated that tensile fracture toughness generally increased with increasing bone size. We also found that fracture toughness of the tissue was significantly related to radiogrammetric indices and that some of these indices explained a greater variability in toughness than porosity, age or gender.

The effects of cortical bone viscoelasticity on the short-term fixation of press-fit cylindrical intramedullary rods

Cementless femoral implants have demonstrated widespread clinical success, particularly in the patient populations for which various cemented techniques have been troublesome [1–4]. Long-term fixation and consequent clinical stability occurs primarily via bony ingrowth into a porous-coated implant surface. The adequacy of this biologic fixation depends in part upon the initial or short-term fixation of the implant with respect to the adjacent bone [5,6]. Short-term fixation refers to the post-operative limitation of relative motion between the porous-coated implant surface and the adjacent bone structure. This relative motion, or micromotion, may be limited by utilizing a porous implant coating in concert with a stem press-fit — both increase the frictional resistance to motion [7]. The holding power of the press-fit over time is dependent upon the viscoelastic nature of cortical bone. Data has long been available in the literature for the viscoelastic behavior of cortical bone in the longitudinal direction [8]; however, a transverse viscoelasticity model is required to evaluate a press-fit since it generates considerable radial and circumferential stress but very little axial stress. Only recently has such a model become available [9].© 2002 ASME