Harry B. Skinner

Age-related decline in proprioception.

Joint-position sense of the knee was measured in 29 subjects with normal knee joints ranging in age from 20 to 82 years. Joint-position sense was determined by two common techniques that measure the threshold to detection of motion and the ability to reproduce passive knee positioning. Joint-position sense was found to deteriorate with increasing age as measured by both tests, with a correlation coefficient that was significant at the p < 0.001 level for each test. The two tests were found to correlate at the p < 0.025 level, indicating that the same biologic parameter was being measured by both tests. Deterioration of proprioception, or joint-position sense, as measured may be a sensitive indicator of subclinical degenerative joint disease of the knee, as well as a means of quantitating proprioception in suspected neuropathic joints.

Ceramic bearing surfaces.

Despite more than 25 years of clinical experience with ceramic materials as bearing surfaces, their role in modern joint replacement surgery remains to be clearly defined. The two primary materials are alumina and zirconia. The application of these materials is primarily as a femoral head bearing surface against polyethylene, but alumina also is used as a femoral head and an acetabulum to provide a polyethylene-free bearing surface. Important issues that must be clarified for these materials to gain wide acceptance are the material properties, wear rates against polyethylene and alumina, the biologic response to ceramic wear debris, and cost in relation to indications. The bulk materials are biocompatible, hard, wettable, high-strength, and can yield good surface finishes. Linear polyethylene wear against alumina heads is reported to be as much as a factor of 5 to 10 lower than metal versus polyethylene. Thus, the ceramic femoral head may be a good choice for the younger patient in whom it seems necessary to use a larger head for stability reasons with a polyethylene cup. Ceramic-on-ceramic wear rates are in the range of 0.003 mm/year, a factor of 10 less than the lowest polyethylene wear rates. These costly materials are limited in head and neck sizes because of statistical variation in strength that can lead to fracture. Occasional reports of high alumina-on-alumina wear have appeared. Many of the problems of the past have been design, manufacture, or application related, and have been improved or eliminated. Proper clinical technique in the use of ceramic femoral heads is crucial to prevent fracture. The materials hold high promise and should continue to be used so that additional experience can help define the clinical indications for components made of these materials.

Complications related to modularity of total hip components.

We report complications from the use of modular components in 20 hip replacements in 18 patients. Fifteen complications (in 13 patients) were related to failure of a modular interface after operation. Femoral head detachment from its trunnion was seen in 6 hips from trauma (3), reduction of a dislocation (2), and normal activity (1). In one case the base of the trunnion fractured below an extra-long modular head. In seven other hips the modular polyethylene liner dislodged from its shell, causing severe damage to the shell in four cases with extensive metallosis. In one other hip an asymmetrical polyethylene liner rotated, resulting in impingement of the femoral component and recurrent dislocation. Operative errors were seen in five cases: implantation of a trial acetabular component in one; and mismatching between the size of the femoral head and the acetabular component in the others. Surgeons who use hip replacements with modular components should be aware of the potential for operative error and of the importance of early treatment for postoperative mechanical failure.

Composite Technology for Total Hip Arthroplasty

Composite materials, which can be very strong while having a low modulus of elasticity, are being studied because such materials have potential to be made into isoelastic hip prostheses. Composites intended for medical applications incorporate carbon or polyamide as a fiber component, while polysulfone, polyetheretherketone, or polyethylene is used as a matrix component. Mechanical properties (especially the modulus of elasticity) are emphasized because of the desire to match those properties of the proximal femur. Many of the variables that affect the mechanical properties of these materials are explained. The application of stress to different fiber orientations demonstrates the mechanical properties of the composite, and this is proved mathematically. It is shown that in composites with fibers oriented in the same direction, the modulus of elasticity in the direction of the fibers generally approaches that of the fibers as the amount of matrix decreases. Perpendicular to the fibers, the modulus of elasticity of the composite is only slightly greater than that of the matrix material. For isotropic chopped-fiber composites, the modulus of elasticity approaches that of the matrix as the fiber content decreases; at high-fiber content, the modulus is significantly less than that of oriented long-fiber composites. In general, the modulus of elasticity and fiber content have a linear relationship. Composites have fatigue properties that vary with direction and approach ultimate strength in tension but are lower in compression. The fatigue properties of proposed composites are discussed. Abrasion as a cause of stress concentration sites and wear particles is considered.

Total elbow arthroplasty. An 18-year experience.

In 34 primary and 18 revision total elbow arthroplasties (TEAs), rheumatoid arthritis (RA) was the most common indication, with traumatic arthritis next most common. The indications were primarily pain and decreased range of motion. Follow-up time averaged 7.6 years for index arthroplasties. Complication rates of primary arthroplasties were high, with 40% neural problems, 9.3% aseptic loosening, and 13.3% early postoperative infection. Overall function was satisfactory in 68% of patients with RA. In agreement, the rates of revision were high. Overall satisfaction was good, and function was fair in patients with TEA.

Pathokinesiology and Total Joint Arthroplasty

Gait analysis data relating to total joint arthroplasty were reviewed to assess their impact on the evolution of prosthetic design. Although joint replacement designs have led to clinical improvement, they do not yet permit the restoration of normal gait. Normal function may be difficult to attain in patients with arthritic destruction, because of a proprioceptive defect. Arthroplasty improves gait by relieving pain, but other factors (previous gait patterns, prosthetic design, muscle weakness, balance, and proprioception) seem to prevent most patients from regaining normal gait.

DECREASED PAIN WITH LOWER FLEXURAL RIGIDITY OF UNCEMENTED FEMORAL PROSTHESES

One hundred one consecutive uncemented hip arthroplasties (87 patients) were analyzed radiographically at 1-year follow up to relate mechanical factors to hip pain as determined by clinical pain scores. The average area moment of inertia and flexural rigidity were greater for the bone than the metal prosthesis for each type of prosthesis (AML, HG, PCA). Normalization of the flexural rigidity ratio (bone to prosthesis) for patient weight yielded a Spearman correlation coefficient of 0.232, significant at P = .02, suggesting that both applied stress and bending stiffness have an effect on pain. No relationship was seen between pain and AP, or average gap between prostheses and bone.

Ankle weight effect on gait: orthotic implications.

Oxygen consumption during ambulation was measured in 10 normal subjects wearing ankle weights of 0.91 kg, 1.82 kg, and 2.73 kg, either on the right ankle or bilaterally. Subjects walked at self-selected speeds and oxygen consumption was measured over 1-minute intervals during steady-state walking. Oxygen consumption per unit distance and oxygen consumption rate demonstrated significant positive linear correlations with added weight (P = .001, P = 0.007, respectively). Velocity demonstrated a significant decrease when correlated with added weight (P = 0.03). Multiple regression analysis was used to relate these measures of oxygen consumption rates to velocity, age, and added weight, yielding predictive relationships. Based on these results, the weight of orthoses should be minimized in order to maximize walking velocity and minimize oxygen consumption per unit distance. The advantage of a light-weight, molded plastic ankle-foot orthosis (AFO) over heavier AFO designs is demonstrated by this study.

Soft tissue sarcoma associated with aluminum oxide ceramic total hip arthroplasty. A case report.

Malignant tumors around fracture fixation implants have been reported sporadically for many years. Recently, however, reports of sarcomatous degeneration around a standard cemented hip arthroplasty and around cobalt-chromium-bearing hip arthroplasties raise new questions of the malignant potential of metallic ends prostheses. Sarcomatous changes around aluminum oxide ceramics seem not to have been reported in the literature. The present report may be the first documented case of an aggressive soft tissue sarcoma detected 15 months after the patient had an uncemented ceramic total hip arthroplasty. If a causal relationship exists, the incidence of this phenomenon in the United States is 250 times greater than would be expected from statistics on soft tissue sarcoma at the hip. Because of the similarity on plane roentgenograms of this tumor to lesions known to be caused by wear debris, tumors should be included in the differential diagnosis of cases of total hip loosening.

ISOELASTICITY AND TOTAL HIP ARTHROPLASTY

Important parameters in designing an isoelastic prosthesis that transfers stress in a more physiologic manner are explored. The effect of material properties and geometry on the mechanical properties of a femoral component are discussed. The bending stiffnesses of cortical bone, metal prosthese, and composite prostheses are compared, and the bending rigidities of prosthesis and bone as they might vary with surgical technique are explained. It is shown that the flexural rigidity of a prosthesis stem as presently designed is close to the flexural rigidity of cortical bone, but 200 to 400 times greater than that of the material it replaces, ie, cancellous bone and fatty marrow.