Biomechanics of the Birmingham hip resurfacing arthroplasty.

The effects of the method of fixation and interface conditions on the biomechanics of the femoral component of the Birmingham hip resurfacing arthroplasty were examined using a highly detailed three-dimensional computer model of the hip. Stresses and strains in the proximal femur were compared for the natural femur and for the femur resurfaced with the Birmingham hip resurfacing. A comparison of cemented versus uncemented fixation showed no advantage of either with regard to bone loading. When the Birmingham hip resurfacing femoral component was fixed to bone, proximal femoral stresses and strains were non-physiological. Bone resorption was predicted in the inferomedial and superolateral bone within the Birmingham hip resurfacing shell. Resorption was limited to the superolateral region when the stem was not fixed. The increased bone strain observed adjacent to the distal stem should stimulate an increase in bone density at that location. The remodelling of bone seen during revision of failed Birmingham hip resurfacing implants appears to be consistent with the predictions of our finite element analysis.

Evaluation of double-exposure holographic interferometry for biomechanical measurements in vitro.

Double-exposure holographic interferometry is a nondestructive testing technique for measuring displacement and strain in a test object. A standard hologram contains three-dimensional information about an object. However, the holographic interferogram has additional information, as a series of interference bands overlaid on the three-dimensional image of the object contains information about object deformation. Interferograms were produced for intact cadaveric femora and cadaveric femora with implanted titanium alloy and cobalt-chromium alloy femoral components. A force was applied to the femoral head to simulate single leg stance, and changes in specimen deformation were observed as additional incremental loads were applied. We have observed that the femur behaves as a bending beam and that the holographic technique allows the position of maximal deflection to be identified and the magnitude of femoral displacement from the load axis to be determined at any point within the field of view. The effects of the modulus of the implanted stem on the bending characteristics of the composite structure were clearly seen in the interferograms. This communication presents a photographic analysis of the double exposure interferograms recorded, as well as a critique of the technique for biomechanical measurements in vitro.

Augmentation of anterior cruciate ligament reconstruction in dogs with prostheses of different stiffnesses.

The purpose of this study was to investigate the biological and biomechanical properties of anterior cruciate ligament (ACL) reconstruction augmented with Dacron prostheses of three different stiffnesses. The ACLs of 36 adult mongrel dogs were removed and the ligament was reconstructed. In 18 dogs, one knee was reconstructed with patellar tendon alone, and the contralateral knee with Dacron augmented patellar tendon. In the remaining 18 dogs, reconstruction was with Dacron augmented patellar tendon with Dacron alone being used for the contralateral control knee. Death was 3 months after surgery, and the reconstructions were examined biologically and biomechanically. The mechanical data were compared with immediate postoperative data obtained from 45 reconstructed fresh cadaveric knees. Tensile testing demonstrated that an increase in failure load was found when the implanted patellar tendon graft was compared with the cadaveric reconstruction. The strength of the Dacron augmented reconstruction showed little change while the Dacron alone graft decreased in strength during the period of implantation. No clear difference was found between the performance of augmentation devices of different stiffnesses. Microangiography showed that grafts were totally revascularized in patellar tendon alone, but not well revascularized in Dacron augmented patellar tendon and Dacron alone reconstruction. The presence of the Dacron appeared to have an adverse effect on revascularization.

Effects of low-modulus coatings on pin-bone contact stresses in external fixation.

The intent of this study was to investigate the stress distribution in cortical bone around fracture fixation pins and around pins coated with various polymeric and elastomeric materials. Since these interface stresses cannot be measured directly, a photoelastic technique was employed and stresses were measured in two-dimensional bone models fabricated from sheets of epoxy resin. Our results showed that when a fixation pin was loaded in compression, the compressive stress measured in the model was greatest at the pin-model interface. The magnitude of the compressive stress was found to diminish steeply away from the hole in a log decrement distribution which was asymptotic to the value of the average stress in the model. When polymeric and elastomeric materials were applied as pin coatings and the performance of the coated pins was compared to that of uncoated pins of the same overall diameter, a reduction of the maximum stress in the bone model was demonstrated. Among the coatings tested, we found that of the polymeric materials ultrahigh molecular weight polyethylene (UHMWPE) was most effective at reducing the peak cortical stress magnitude. The most effective coating material overall was found to be silicon elastomer. Computation of stress values in models loaded through stainless-steel pins and through pins coated with 1-mm silicon elastomer showed that the presence of the elastomer layer caused a reduction of about 50% in the maximum compressive stress in the model.

Remodeling of bone around hydroxyapatite-coated femoral stems.

Two hundred and twenty-four total hip arthroplasties were performed in 201 patients with use of a femoral component with hydroxyapatite coating of the proximal portion of the stem. The mean duration of follow-up was seventy-one months (range, fifty-eight to eighty-seven months). Of the 224 arthroplasties, 208 (93 per cent; 190 patients) yielded a good or excellent clinical result. Four patients (2 per cent) reported mild-to-moderate activity-related pain in the thigh, and two (1 per cent) had aseptic loosening. The radiographic findings of progressive new-bone formation (cancellous condensation and cortical hypertrophy) throughout the zones adjacent to the middle and distal portions of the stem were evidence of early, extensive proximal fixation of the implant, with distal stress transfer through the implant, which is stiffer than the surrounding bone. Remodeling of the femur began early, was predictable, and progressed throughout the follow-up period. Cortical hypertrophy about the middle and distal portions of the stem occurred predominantly in the mediolateral plane (in 105 hips [47 per cent], compared with thirteen hips [6 per cent] in the anteroposterior plane), and it was more common in patients who had had poorer bone quality preoperatively. Intramedullary osteolysis was present in one femur (0.4 per cent) at five years; the osteolytic area was less than five millimeters in its greatest dimension and had not progressed at the time of the six-year follow-up evaluation. This low rate of osteolysis suggests that a circumferential coating of hydroxyapatite may effectively minimize migration of wear debris along the femoral stem. The progressive remodeling of the femur about the middle and distal portions of the stem, as evidenced by cancellous condensation and cortical hypertrophy, has not, to our knowledge, been described previously to this magnitude in association with proximally coated (porous or hydroxyapatite-coated) femoral implants.

Early clinical experience with hydroxyapatite-coated femoral implants.

As part of a multi-center study, 238 titanium stems that were proximally coated with hydroxyapatite were implanted in 220 patients between January 1988 and December 1989. Ninety-two of these stems in eighty-three patients had a minimum of two years of follow-up, including analysis of the clinical and radiographic data. Clinically, the patients were essentially pain-free before six months and had a low (4 per cent) prevalence of pain in the thigh and a very high composite Harris hip-score (mean, 95 points) at two years. Radiographically, subsidence was detected in 8 per cent of the implants; no implant had more than three millimeters of subsidence. Radiolucencies were characteristically seen around the uncoated distal part of the stem, in 70 per cent of the implants. Contrastingly, radiolucencies were rare in the hydroxyapatite-coated proximal zones and were most often found anteriorly, in only 5 per cent of the implants. Areas of increased formation of cancellous bone were seen beneath femoral cortical bone at the interface between the hydroxyapatite-coated and uncoated parts of the stem, in 67 per cent of the implants. Calcar resorption was found in 49 per cent of the implants. Cortical thickening was present in 17 per cent of the implants at the uncoated distal part of the stem. Two of the 238 femoral implants were revised: one because of infection and one because of aseptic loosening associated with non-union of a subtrochanteric osteotomy.

Hydroxyapatite-coated total hip femoral components in patients less than fifty years old. Clinical and radiographic results after five to eight years of follow-up.

One hundred and thirty-three patients (152 hips) who were an average of thirty-nine years old (range, sixteen to forty-nine years old) received a proximally hydroxyapatite-coated femoral prosthesis as part of a total hip arthroplasty and were followed for a minimum of five years (average, 6.4 years; range, five to 8.3 years) or until revision. The average Harris hip score was 47 points (range, 22 to 77 points) preoperatively and 93 points (range, 49 to 100 points) at the time of the latest clinical evaluation. Two patients who had a well fixed femoral implant had activity-limiting pain in the thigh at the time of the most recent examination. Radiographic changes consistent with bone-remodeling (cortical hypertrophy and bone condensation) typically were seen around the mid-part of the shaft of the prosthesis. Forty-eight (32 per cent) of the 148 hips that were included in the radiographic analysis demonstrated a small amount of erosive scalloping in either zone 1 or zone 7 of Gruen et al., and intramedullary osteolysis was suspected in only one hip. All stems were radiographically osseointegrated according to a modification of the criteria described by Engh et al. Four stems were revised, but none of the revisions were performed because of mechanical failure (two stems were revised in conjunction with a revision of the cup because of pain; one, because of an infection; and one, after a traumatic femoral fracture that occurred six years postoperatively). Thus, the rates of aseptic and mechanical failure were both 0 per cent. The combined rate of failure, which included the two stems that were revised because of pain and the two stems that were associated with pain that limited activity, was 2.6 per cent (four of 152 stems). The over-all clinical results associated with hydroxyapatite-coated femoral components were excellent in this group of young patients after intermediate-term follow-up. A review of serial radiographs showed mechanically stable implants with osseous ingrowth, evidence of stress transmission at the middle part of the stem, and minimum endosteal osteolysis.

Fixation of acetabular cups without cement in total hip arthroplasty. A comparison of three different implant surfaces at a minimum duration of follow-up of five years.

We evaluated 377 patients (428 hips) who had been managed, by a total of fourteen surgeons at twelve clinical sites in the United States and Europe, with a porous-coated press-fit acetabular cup, a hydroxyapatite-coated threaded screw-in cup, or one of two similar designs of hydroxyapatite-coated press-fit cups between April 1987 and November 1992. The same type of hydroxyapatite-coated femoral stem was inserted without cement in all patients. After a minimum duration of follow-up of five years (mean, 7.9 years; range, 5.3 to 9.1 years), one (1 per cent) of the 131 hydroxyapatite-coated threaded cups, two (2 per cent) of the 109 porous-coated press-fit cups, and twenty-one (11 per cent) of the 188 hydroxyapatite-coated press-fit cups had been revised because of aseptic loosening. A common radiographic sign of impending failure of the hydroxyapatite-coated press-fit cups was radiolucency at the interface between the implant and the subchondral bone beneath it. This radiolucency usually was seen initially more than two years after implantation. Radiographic evaluation of the 383 acetabular implants that were in situ at the time of the most recent follow-up showed that 123 (99 per cent) of the 124 hydroxyapatite-coated threaded cups, 101 (98 per cent) of the 103 porous-coated cups, and 139 (89 per cent) of the 156 hydroxyapatite-coated press-fit cups were stable with osseous ingrowth (as indicated by the absence of radiolucency at the interface and the absence of migration within the acetabulum). The probability of revision due to aseptic loosening was significantly greater for the hydroxyapatite-coated press-fit cups than it was for the hydroxyapatite-coated threaded cups or the porous-coated press-fit cups (p < 0.001 for both comparisons). Within the group of patients who had a hydroxyapatite-coated press-fit cup, the probability of revision due to aseptic loosening was significantly greater in association with a young age (p = 0.003), female gender (p = 0.02), the use of a femoral head with a diameter of thirty-two millimeters (p = 0.018), and the use of a thin polyethylene liner (p < 0.001). We found that the hydroxyapatite-coated threaded cups and the porous-coated press-fit cups continued to perform well more than five years after the operation. The hydroxyapatite-coated press-fit cups that were revised probably failed because the fixation interface beneath the cup could not sustain the tensile stresses that were imposed between the cup and the bone by the activity of the patient. Our data suggest that, in the specific biomechanical environment of the acetabulum, physical interlocking between the cup and the supporting bone beneath it may be a prerequisite for long-term stability.

Graft tension in anterior cruciate ligament reconstruction. An in vivo study in dogs.

The purpose of this study was to examine the effect of initial tensioning on the outcome of reconstruction of the ACL. The ACLs of 15 adult mongrel dogs were excised and reconstructed. In the first five dogs, the ACLs of both knees were reconstructed using the medial one-third of the patellar tendon. The graft was fixed under a tension of 1 N (0.22 pounds) in one knee and 39 N (8.8 pounds) in the opposite knee. In the remainder of the dogs, the reconstructions were augmented with Dacron prostheses. Tensioning of both graft components in the augmented reconstructions was either with 1 N in one knee and 39 N in the contralateral knee or disproportionate tensions of 1 N and 39 N applied to the autogenous material and to the prosthesis. Sacrifice was 3 months postsurgery and results were examined with microangiography/history and mechanical testing. In the reconstructions with the patellar tendon alone, the biologic study showed poor vascularity and focal myxoid degeneration within the graft pretensioned with a load of 39 N. In the augmented reconstructions, the knees in which both graft materials were fixed with 1 N tension showed the strongest and stiffest reconstruction at 3 months. The study suggests that minimal tension should be applied to the graft materials during surgical reconstruction of the ACL.

A ported, proximally-cemented femoral stem for total hip arthroplasty. Development and clinical application.

We describe the development and early clinical application of a ported, proximally-cemented titanium stem for cemented total hip arthroplasty. PMMA bone cement is delivered to the proximal femur under pressure after the stem has been positioned within the femoral canal. A mid-stem cement occluder contains the cement to the proximal stem only. A tapered body is incorporated in the design of the stem to reduce the structural stiffness and hence the degree of stress shielding within the reconstructed joint. We performed preclinical studies to measure the reduction in porosity and the pressurisation achieved. The porosity, as measured by the void percentage within the cured cement mantle, was reduced by more than 50% and there was an almost threefold increase in the mean pressure. Mechanical testing of the stem, using a three-point bend test, showed that the addition of cement injection ports on the anterior and posterior sides of the body of the proximal stem did not reduce its strength. Finite-element analysis indicated that, compared with a fully-cemented conventional stem, there was no change in the stresses within the cement mantle. In a series of 40 proximally-cemented stems followed for up to six years (mean 51 months) the mean Harris hip score was 91, and 85% of patients had good or excellent results. There was excellent pain relief, an increased level of activity and good patient satisfaction. One mechanical failure of the stem required revision at three years after implantation. The early results indicate that the clinical performance was equal to that achieved with other modern cemented stems. Radiological evaluation showed excellent results with no evidence of stress shielding. Further follow-up will determine if long-term stress shielding is reduced and if revision is made easier by the absence of a distal cement mantle.

Clinical results with the hydroxyapatite-coated Omnifit hip stem.

A 2-year follow-up from a prospective clinical trial investigation of the hydroxyapatite (HA)-coated Omnifit-HA Hip Stem is presented. Assessment of clinical and functional evaluation parameters demonstrates the excellent results obtained with the femoral stem. Radiographic observations indicate a complete lack of fibrous tissue in the HA-coated regions of the stem, thus promoting direct load transfer between the stem and bone. Comparison with published studies suggests that the clinical results obtained with the HA-coated Omnifit-HA Hip Stem compare favorably with the prior reports.

The clinical assessment of the normal and abnormal foot during locomotion.

The clinical assessment of the weight-bearing foot during locomotion is normally based on subjective judgement rather than on quantitative measurement. Although anatomical abnormalities are often apparent at examination, the accurate assessment of an abnormality of function is more difficult to assess particularly if the abnormality is only apparent under dynamic loading conditions. The many drawbacks in previous methods proposed for the clinical assessment of gait have led to the development of a novel system which allows an immediate quantitative visualization of the magnitude and point of application of the forces applied to the plantar surface of the foot during locomotion. This paper describes the technique and presents visual data on normal locomotion, on abnormal locomotion and the changes induced into a patient's abnormal gait by corrective surgery.

Uncemented stems in hip replacement--hydroxyapatite or plain porous: does it matter? Based on a prospective study of HA Omnifit stems at 15-years minimum follow-up.

For many years, acrylic cement has been regarded as the unique available means for a long term and secure fixation of components in hip arthroplasty. A new generation of uncemented implants coated in hydroxyapatite (HA) has arisen since the mid-1980s, aiming to provide a 'biological interface' between metal and surrounding bone, and thus the hydroxyapatite interface was defined some years ago as a distinct entity from both cemented and 'plain porous' fixation. Based upon our 20-year experience with the HA Omnifit stem, this paper aims to discuss the efficiency of hydroxyapatite as a means of fixation for femoral components in hip arthroplasty, then examine whether the addition of a calcium phosphate layer induces any adverse effects, and finally make comparisons between HA-coated versus porous hip stems reported in the literature. With respect to fixation of femoral components in hip arthroplasty we report excellent results from the partially coated HA Omnifit stem in our series, with 99.20% of survival rate at 17-year follow-up, these results being consistent and similar to other HA series in the literature. HA 'uncemented' fixation can therefore be considered reliable and efficient. Furthermore, two decades of hydroxyapatite coatings have resulted in the identification of no major adverse effects. In fact calcium phosphate ions participate in the physiological turn-over of bone remodelling, and the HA coating is replaced by new bone formation without any fibrous tissue layer. Since HA particles are biodegradable and do not produce any inflammatory reaction in the surrounding bone, fears of osteolysis or third body wear due to HA debris have not been confirmed. Finally, comparison between HA versus plain porous femoral components through the literature has demonstrated better results with HA than porous alone both in terms of the quantity and quality of bone remodelling, and the potential migration and subsidence of the stem.

Hydroxyapatite-coated total knee replacement: clinical experience at 10 to 15 years.

This study describes 146 primary total knee replacements, either fully or partially coated with hydroxyapatite of which 74 knees in 68 patients were available for clinical and radiological assessment at a mean of 11.2 years (10 to 15). The global failure rate was 1.37% and survival rate with mechanical failure as the end-point was 98.14%. Radiological assessment indicated intimate contact between bone and the hydroxyapatite coating. Over time the hydroxyapatite coating appears to encourage filling of interface gaps remaining after surgery. Our results compare favourably with those of series describing cemented or porous-coated knee replacements, and suggest that fixation with hydroxyapatite is a reliable option in primary total knee replacement.

Incidence, contributory factors and costs of pressure sores.

Although pressure sores are an increasing problem in long-term medical care, the magnitude and costs of the complication are not well documented. In this article an investigation of the pressure sore problem in a general hospital is described. The incidence of patients with pressure sores was found to be 4,5% of the total patient population, with an additional 5,2% showing signs of skin discoloration from excessive or prolonged pressure. The majority of patients with sores were over 60 years of age, incontinent, immobile and White.

Wear debris. An environmental issue in total joint replacement.

There is a growing concern that osteolytic lesions, often adjacent to otherwise stable implants, are a recent phenomenon caused by some recent change in polyethylene, metal, or other aspect of the total hip construction. This study investigates the possibility that bearings and modular connections used in modern hip replacements are an unappreciated source of particulate debris. Measurements taken from contemporary femoral bearings show a significant mismatch in both surface finish and sphericity of mating metal and polyethylene components, with sphericity of inserts being much worse then sphericity of femoral heads. The tolerances for sphericity of polyethylene inserts were further changed by the placement of an insert into its metal shell. Hip simulator tests of assembled inserts and shells showed greater polyethylene weight loss for metal-backed shells than for inserts alone. Bending and torsional tests of metal/metal modular connections showed that dynamic loads can release large numbers of debris particles from taper junctions. Because osteolytic lesions clearly are associated with overload of tissue by debris particles, the design, manufacture, and tolerances of modular connections in total hip replacement all seem to require reevaluation.

Repetitive mechanical stress and denervation in plantar ulcer pathogenesis in rats.

To test the hypotheses that physiologic limits of repetitive mechanical stress can stimulate the formation of ulcers on the plantar surface of the foot, and that denervation predisposes to such ulcer formation, 90 rats were subjected to neurectomy or tenotomy and the response of each animal's hind foot to 10,000 daily repetitions of mechanical stress in a walking simulator was determined. Plantar ulceration occurred in the neurectomized animals within 7 to 10 days of commencing the simulated walking procedures. Histologic examination of all the stimulated footpads showed traumatic damage in both neurectomized and tenotomized animals. It was concluded that normal levels of repetitive mechanical stress can cause plantar ulceration and that such ulceration occurs more readily in the denervated foot.

The vertical wheeler: a device for ambulation in cerebral palsy.

The vertical wheeler is a new mobility aid that was specifically designed to help improve the quality of life for the handicapped child by providing mobility while standing. Results of a clinical trial in a population of patients with cerebral palsy are presented. Criteria were selected to allow evaluation of the rehabilitative effect of the device on the population. Results showed that the children in this cerebral palsy group all benefited from ambulation with the wheeler. Patients with spastic quadriparesis seemed to gain the most immediate benefit. The device contributed to improved mobility, posture, and self-image. The wheeler was safe and fun for the children. It has the potential for improving the psychologic and medical status of the child with severe locomotion impairment.