Porous Ingrowth Surface Research Articles

Humeral stem loosening is not always prosthetic joint infection

BackgroundAseptic humeral stem loosening is an uncommon complication of shoulder arthroplasty, and its presence has been considered a highly specific predictor of prosthetic joint infection (PJI). Literature on aseptic humeral stem loosening is sparse. The primary purpose of this study was to determine the rate of aseptic humeral loosening in revision shoulder arthroplasty. Secondarily, we sought to identify predictors of septic and aseptic humeral loosening. MethodsOur institutional revision shoulder arthroplasty database was reviewed. Inclusion criteria were patients with humeral stem loosening as indicated in the operative report. Patient demographics, index surgery and indication, revision surgery and indication, and operative data were recorded. Charts were manually reviewed, and PJI scores were calculated using the International Consensus Meeting (ICM) scoring criteria. ICM classifications of PJI “unlikely” and “possible” were considered aseptic, and PJI “probable” and “definite” were considered septic. Statistical analysis was performed to determine associations between the abovementioned variables and aseptic humeral loosening. ResultsForty-six patients with forty-eight shoulders were included in our analysis. Ten cases (21%) were classified as definite PJI, thirteen cases (27%) were classified as probable PJI, four cases (8%) were classified as possible PJI, and twenty-one cases (44%) were classified as PJI unlikely. On analysis of stem design, there were no statistically significant associations with aseptic loosening; although, a lack of proximal porous ingrowth surface trended toward higher rates of aseptic loosening in all patients and in ATSA and RTSA subgroup analyses. In the index RTSA subgroup, aseptic loosening was associated with female sex (p=0.005). Seventeen of 39 shoulders (44%) that underwent either index ATSA or RTSA demonstrated concomitant glenoid loosening. The absence of glenoid loosening was associated with aseptic humeral loosening in index ATSA and RTSA (p<0.001). Conclusion52% of revision shoulder arthroplasty cases with humeral loosening performed at our institution were aseptic. There appear to be distinct demographic and radiographic factors that are more commonly associated with aseptic as opposed to septic humeral loosening. Our data demonstrate that demographic predictors of aseptic loosening of RTSA include female sex. The absence of glenoid component loosening is associated with aseptic humeral loosening and concomitant glenoid loosening is associated with septic humeral loosening. Understanding of these factors can guide the preoperative index of suspicion for PJI in the setting of humeral stem loosening.

Cementless Reverse Shoulder Arthroplasty Technique to Maximize Press-Fit Fixation with Humeral Matchstick Bone Grafts.

Cementless reverse shoulder arthroplasty has become increasingly popular because of the improved implant design, porous ingrowth surface, and surgical techniques. When avoiding the risks of cement use, a press-fit arthroplasty stem that has been implanted may not feel immediately stable, especially if the medullary canal size is in between standard stem diameters. To help surgeons improve fixation and avoid overstuffing the medullary canal, we present the matchstick autograft augmentation technique. The use of humeral autograft, analogous to impaction grafting in hip arthroplasty, has been reported to have promising short-term outcomes2,3. This technique of using humeral autograft material, dubbed matchstick autografts because of their shape and size, allows for optimization of humeral stem stability with the option of smaller cementless humeral implants. By avoiding overstuffing of the medullary canal, this technique aims to reduce the incidences of intraoperative fracture, postoperative stress shielding, and potential implant loosening4-6. Cementless reverse total shoulder arthroplasty is routinely performed via the anterosuperior approach7; however, a deltopectoral approach can be utilized if desired. The canal is sequentially broached with implant trials until the tactile feedback demonstrates axial and rotational stability. In cases in which tactile feedback during implantation demonstrates slight movement, the smaller implant size can be selected and augmented with matchstick autograft. An oscillating saw is utilized to cut the edges of the previously resected humeral head in order to expose the subchondral bone surface. Graft sticks about 20 mm in length and 1 to 3 mm in width are then fashioned. Humeral trials are then implanted with the matchstick grafts placed lengthwise alongside the humeral stem. Axial and rotational press-fit is again assessed. If adequate, the formal humeral implant is selected and implanted in position. As in conventional impaction grafting, the grafts are compressed to the side of the humeral canal, but they offer more corticocancellous structure than bone chips. This technique is applicable even in some fracture scenarios. When a specific press-fit humeral stem size does not achieve adequate stability, there are typically 3 surgical alternatives. First, a larger stem size can be selected. Second, the implant can be inserted deeper to achieve press-fit stability. Third, cement can be added to fill the medullary canal and create immediate stability. When implanting the humeral prosthesis, the operating surgeon's primary goal is stem stability. When faced with lack of stability, the surgeon can select a larger humeral stem, risking stress shielding; implant the stem deeper, compromising length and risking humeral fracture; or consider a cemented implant. In order to minimize the risk of intraoperative cardiopulmonary events and complicated subsequent revision surgeries8, the use of cement should be avoided if at all possible. Shoulder surgeons have reported grafting techniques, analogous to hip impaction grafting, that have yielded good success3. The technique that we describe utilizes a matchstick structural autograft that helps improve cementless fixation in primary humeral implantation cases and allows for the use of a smaller stem. The structural shape of the graft allows this technique to be utilized even in selected proximal humeral fractures. Other studies have reported on the use of softer cancellous autografts to stabilize humeral implants in shoulder arthroplasty. In a study of 286 arthroplasties with a minimum follow-up of 2 years, Lucas et al. reported that 267 humeral stems (93.3%) had not subsided3. Humphrey and Bravman used cancellous autograft to achieve metaphyseal centering of the humeral component in 53 patients, with no cases of humeral implant loosening at 12 months2. Lo et al. reported 91% tuberosity healing in their series of cementless reverse total shoulder arthroplasties augmented with matchstick autografts1, with no cases of aseptic humeral stem loosening. Montemaggi et al. used matchstick autografts to augment 46 primary cementless reverse total shoulder arthroplasties and found zero instances of humeral loosening at 1-year follow-up9. The strongest humeral matchstick grafts come from the subchondral surface.After creating the graft, it is palpated for structural integrity. A stiffer or softer graft can be chosen, depending on surgeon preference.Surgeons can try impacting the graft with humeral trials to assess the stem stability prior to final implantation. RTSA = reverse total shoulder arthroplastyFX = fracture3D CT = 3-dimensional computed tomographyXR = x-rayFU = follow-up.

A NOVEL 3D-PRINTED DUAL-MOBILITY IMPLANT: RESULTS UP TO SEVEN YEARS IN CLINICAL PRACTICE

Dual mobility (DM) is an established bearing option in Total Hip Arthroplasty (THA). The traditional mono-block DM designs have limited ability for additional fixation, whereas the modular DM designs allow additional screw fixation but limit internal diameter and have the potential to generate metal debris. We report the early results of a CoCrMo alloy mono-block implant manufactured by additive technology with a highly porous ingrowth surface to enhance primary fixation and osseointegration.Prospective follow-up of the DuplexTM implant first inserted in March 2016 enrolled into Beyond Compliance (BC). Primary outcome measure was all-cause revision and secondary outcomes dislocation, peri-prosthetic fracture (PPF) and Oxford Hip Score (OHS). Patients were risk stratified and all considered to be high risk for instability. Complications were identified via hospital records, clinical coding linkage using national database and via BC website.159 implants in 154 patients with a mean age 74.0 years and a maximum F/U of 7 years. Survivorship for all-cause revision 99.4% (95% CI 96.2–99.8). One femoral only revision. Mean gain in OHS 27.4. Dislocation rate 0.6% with a single event. Patients with a cemented Polished taper stem (PTS) had a Type B PPF rate of 2.1% requiring revision/fixation. Compared to conventional THA this cohort was significantly older (74.0 vs 68.3 years), more co-morbidity (ASA 3 46.5% vs 14.4%) and more non-OA indications (32.4% vs 8.5%). Every patient had at least one risk factor for falling and &gt;50% of cohort had 4 or more risk factors using NICE tool.We believe our results demonstrate that risk stratification successfully aids implant selection to prevent dislocation in high-risk patients. This novel design has provided excellent early results in a challenging cohort where individuals are very different to the “average” THA patient. NJR data on DM has reported an increase in revision for PPF. A “perfect storm” maybe created using DM in high-risk falls risk population. This re-enforces the need to consider all patient and implant factors when deciding bearing selection.

COMPARATIVE SURVIVAL OF CONTEMPORARY CEMENTLESS ACETABULAR COMPONENTS FOLLOWING PRIMARY TOTAL HIP ARTHROPLASTY

The last two decades have seen remarkable technological advances in total hip arthroplasty (THA) implant design. Porous ingrowth surfaces and highly crosslinked polyethylene (HXLPE) have been expected to dramatically improve implant survivorship. The purpose of the present study was to evaluate survival of contemporary cementless acetabular components following primary THA.16,421 primary THAs performed for osteoarthritis between 2000 and 2019 were identified from our institutional total joint registry. Patients received one of 12 contemporary cementless acetabular designs with HXLPE liners. Components were grouped based on ingrowth surface into 4 categories: porous titanium (n=10,952, mean follow-up 5 years), porous tantalum (n=1223, mean follow-up 5 years), metal mesh (n=2680, mean follow-up 6.5 years), and hydroxyapatite (HA) coated (n=1566, mean follow-up 2.4 years). Kaplan-Meier analyses were performed to assess the survivorship free of acetabular revision. A historical series of 182 Harris-Galante-1 (HG-1) acetabular components was used as reference.The 15-year survivorship free of acetabular revision was &gt;97% for all 4 contemporary cohorts. Compared to historical control, porous titanium (HR 0.06, 95% CI 0.02–0.17, p&lt;0.001), porous tantalum (HR 0.09, 95%CI 0.03–0.29, p&lt;0.001), metal mesh (HR 0.11, 95%CI 0.04–0.31, p&lt;0.001), and HA-coated (HR 0.14, 95%CI 0.04–0.48, p=0.002) ingrowth surfaces had significantly lower risk of any acetabular revision. There were 16 cases (0.1%) of acetabular aseptic loosening that occurred in 8 (0.07%) porous titanium, 5 (0.2%) metal mesh, and 3 (0.2%) HA-coated acetabular components. 7 of the 8 porous titanium aseptic loosening cases occurred in one known problematic design. There were no cases of aseptic loosening in the porous tantalum group.Modern acetabular ingrowth surfaces and HXLPE liners have improved on historical results at the mid-term. Contemporary designs have extraordinarily high revision-free survivorship, and aseptic loosening is now a rare complication.At mid-term follow-up, survivorship of contemporary uncemented acetabular components is excellent and aseptic loosening occurs in a very small minority of patients.

Comparative Survival of Contemporary Cementless Acetabular Components Following Revision Total Hip Arthroplasty

BackgroundThe advent of highly porous ingrowth surfaces and highly crosslinked polyethylene has been expected to improve implant survivorship in revision total hip arthroplasty. Therefore, we sought to evaluate the survival of several contemporary acetabular designs following revision total hip arthroplasty. MethodsAcetabular revisions performed from 2000 to 2019 were identified from our institutional total joint registry. We studied 3,348 revision hips, implanted with 1 of 7 cementless acetabular designs. These were paired with highly crosslinked polyethylene or dual-mobility liners. A historical series of 258 Harris-Galante-1 components, paired with conventional polyethylene, was used as reference. Survivorship analyses were performed. For the 2,976 hips with minimum 2-year follow-up, the median follow-up was 8 years (range, 2 to 35 years). ResultsContemporary components with adequate follow-up had survivorship free of acetabular rerevision of ≥95% at 10-year follow-up. Relative to Harris-Galante-1 components, 10-year survivorship free of all-cause acetabular cup rerevision was significantly higher in Zimmer Trabecular Metarevision (hazard ratio (HR) 0.3, 95% confidence interval (CI) 0.2-0.45), Zimmer Trabecular MetaModular (HR 0.34, 95% CI 0.13-0.89), Zimmer Trilogy (HR 0.4, 95% CI 0.24-0.69), DePuy Pinnacle Porocoat (HR 0.24, 95% CI 0.11-0.51), and Stryker Tritanium revision (HR 0.46, 95% CI 0.24-0.91) shells. Among contemporary components, there were only 23 rerevisions for acetabular aseptic loosening and no rerevisions for polyethylene wear. ConclusionContemporary acetabular ingrowth and bearing surfaces were associated with no rerevisions for wear and aseptic loosening was uncommon, particularly with highly porous designs. Therefore, it appears that contemporary revision acetabular components have dramatically improved upon historical results at available follow-up.

Minimum ten-year follow-up of a randomized trial comparing acetabular component fixation of two porous in-growth surfaces using radiosteriometric analysis.

AimsTo compare the in vivo long-term fixation achieved by two acetabular components with different porous ingrowth surfaces using radiostereometric analysis (RSA).MethodsThis was a minimum ten-year follow-up of a prospective randomized trial of 62 hips with two different porous ingrowth acetabular components. RSA exams had previously been acquired through two years of follow-up. Patients returned for RSA examination at a minimum of ten years. In addition, radiological appearance of these acetabular components was analyzed, and patient-reported outcome measures (PROMs) obtained.ResultsIn all, 15 hips were available at ten years. There was no statistically significant difference in PROMS between the two groups; PROMs were improved at ten years compared to preoperative scores. Conventional radiological assessment revealed well-fixed components. There was minimal movement for both porous surfaces in translation (X, Y, Z, 3D translation in mm (median and interquartile range (IQR)), StikTite (Smith and Nephew, Memphis, Tennessee, USA): 0.03 (1.08), 0.12 (0.7), 0.003 (2.3), 0.37 (0.30), and Roughcoat (Smith and Nephew): -0.6 (0.59),–0.1 (0.49), 0.1 (1.12), 0.48 (0.38)), and rotation (X, Y, Z rotation in degrees (median and IQR), (Stiktite: -0.4 (3), 0.28 (2), -0.2 (1), and Roughcoat: - 0.4 (1),–0.1 (1), 0.2 (2)). There was no statistically significant difference between the two cohorts (p-value for X, Y, Z, 3D translation - 0.54, 0.46, 0.87, 0.55 and for X, Y, Z rotation - 0.41, 0.23, 0.23 respectively) at ten years. There was significant correlation between two years and ten years 3D translation for all components (r = 0.81(p =< 0.001)).ConclusionBoth porous ingrowth surfaces demonstrated excellent fixation on plain radiographs and with RSA at ten years. Short-term RSA data are good predictors for long-term migration data.

Primary Stability in Cementless Rotating Platform Total Knee Arthroplasty.

Highly porous ingrowth surfaces have been introduced into tibial tray fixation to improve long-term survivorship in cementless total knee arthroplasty. This study was designed to evaluate the effect of porous ingrowth surface on primary stability in the implanted cementless tibial component. Three tibial tray designs possessing sintered bead or roughened porous coating ingrowth surfaces were implanted into a foam tibia model with primary stability assessed via digital image correlation during stair descent and condylar liftoff loading. Follow-up testing was conducted by implanting matched-pair cadaveric tibias with otherwise identical trays with two iterations of ingrowth surface design. Trays were loaded and micromotion evaluated in a condylar liftoff model. The sintered bead tibial tray exhibited slightly lower micromotion than the roughened porous coating in stair descent loading. However, no significant difference in primary stability was observed in condylar liftoff loading in either foam or cadaveric specimens. Cementless tibial trays featuring two different iterations of porous ingrowth surfaces demonstrated both good stability in cadaveric specimens with less than 80 microns of micromotion and 1 mm of subsidence under cyclic loading. While improved ingrowth surfaces may lead to improved biological fixation and long-term osteointegration, this study was unable to identify a difference in primary stability associated with subsequent ingrown surface design iteration.

The jumbo cup: Curtain calls and caveats

Use of an oversized “jumbo cup” is an effective technique for revision of most failed acetabular components. The jumbo cup is prepared with hemispherical reamers and provides a large porous ingrowth surface area in direct contact with the host bone. However, since the oversized cup is larger than the native acetabulum, the hip center can be raised, which may require use of a longer femoral head to maintain soft tissue tension and leg length. The anterior edge of the cup may also protrude through the anterior wall, which could be a cause of iliopsoas tendonitis resulting in groin pain.

Removal of a well-fixed femoral stem: Tour de force

Revision total hip arthroplasty often requires removal of a well-fixed femoral implant. Ease of implant removal is dependent on the stem design, geometry, and location/extent of porous ingrowth surface. In an effort to preserve the proximal femoral bone, many different surgical exposures and techniques are available. The intent of this review article is to briefly review the most popular techniques and their indications for use revision total hip, well-fixed femoral stem, femoral stem design.

Long-Term Outcome of a Metal-on-Polyethylene Cementless Hip Resurfacing

Due to the well-documented problems surrounding metal-on-metal bearings, the use of hip resurfacing has declined. Since the potential benefits of hip resurfacing remain desirable, it may be beneficial to investigate the long-term outcome of hip resurfacings using metal-on-polyethylene in the 1980’s. We report the long-term survivorship and modes of failure of a cementless metal-on-polyethylene resurfacing (n=178) with different porous ingrowth surfaces. While acetabular loosening was absent, a high incidence of femoral failures (femoral loosening=18.1%, osteolytic neck fracture=21%) occurred despite using the same ingrowth surface for both components. Ongoing developments using the lessons learned from these previous generation components and utilizing modern low wear materials, e.g., cross-linked polyethylene, may lead to improved implants for future hip resurfacings.

A Randomized Trial Comparing Acetabular Component Fixation of Two Porous Ingrowth Surfaces Using RSA

Several new porous ingrowth surfaces for acetabular component fixation have recently been developed. The purpose of this study was to compare the in vivo fixation achieved by two different porosity ingrowth surfaces using radiostereometric analysis (RSA). Sixty-two patients undergoing primary total hip arthroplasty (THA) were randomized to receive a cementless acetabular component with either a 61% high porosity asymmetric titanium porous surface (StikTite, Smith and Nephew, TN, USA) or a 45% low porosity sintered bead porous surface (Roughcoat, Smith and Nephew, TN, USA). RSA and clinical follow-up examinations were done post-operatively, 6-weeks, 3-months, 6-months, 1-year and 2-years. Both the high porosity StikTite and lower porosity Roughcoat surfaces provided excellent biologic fixation.

Ten-Year Outcome of Serum Metal Ion Levels After Primary Total Hip Arthroplasty

We previously reported on the metal ion concentrations of cobalt, chromium, and titanium that were found in the serum of patients three years after they had undergone primary total hip arthroplasty as compared with the concentrations found in the serum of control patients who did not have an implant. This study is a concise update on the serum metal levels found in a cohort of these patients ten years after the time of hip implantation. Of the original seventy-five subjects, metal ion levels were available for forty patients (53%). Ten patients (hybrid group) had received a hybrid total hip replacement that consisted of a modular cobalt-alloy femoral stem with a cobalt-alloy femoral head that had been inserted with cement and a titanium acetabular socket that had been inserted without cement. Nine patients (cobalt-chromium [CoCr] group) had received an implant with an extensively porous-coated modular cobalt-alloy femoral stem and femoral head along with a titanium acetabular socket; the femoral and acetabular components had each been inserted without cement. Eight patients (titanium group) had undergone insertion of a proximally porous-coated modular titanium-alloy femoral stem with a cobalt-alloy femoral head and a titanium acetabular socket; the femoral and acetabular components had each been inserted without cement. Thirteen patients (control group) from the original control group of patients who had not received an implant served as control subjects. Serum metal levels were measured with use of high-resolution sector field inductively coupled plasma mass spectrometry. The hybrid total hip arthroplasty group had mean cobalt levels that were 3.2 times higher at 120 months than they were at baseline, and the cobalt levels in that group were significantly higher than those in the titanium total hip arthroplasty group at thirty-six, sixty, eighty-four, ninety-six, and 120 months (p < 0.01). The hybrid group had mean chromium levels that were 3.9 times higher at 120 months than they were at baseline, and the CoCr total hip arthroplasty group had chromium levels that were 3.6 times higher at 120 months than they were at baseline. The serum titanium levels were higher in the titanium group at all follow-up time intervals as compared with the levels in all other groups, and the level in the titanium group at 120 months was eighteen times higher than it was at baseline (p < 0.01). Patients with well-functioning primary metal-on-polyethylene total hip replacements had elevated serum metal levels for as many as ten years postoperatively. Furthermore, metal release at the modular femoral head-neck junctions, rather than passive dissolution from porous ingrowth surfaces, was likely the dominant source of serum cobalt and chromium.

Acetabular Impaction Grafting in Total Hip Replacement

Acetabular impaction grafting has been shown to be very effective, but concerns regarding its suitability for larger defects have been highlighted. We report the use of this technique in a large cohort of patients, and address possible limitations of the technique. We investigated a consecutive group of 339 cases of impaction grafting of the cup with morcellised impacted allograft bone for survivorship and mechanisms for early failure. Kaplan Meier survival was 89.1% (95% CI 83.2 to 95.0%) at 5.8 years for revision for any reason, and 91.6% (95% CI 85.9 to 97.3%) for revision for aseptic loosening of the cup. Of the 15 cases revised for aseptic cup loosening, nine were large rim mesh reconstructions, two were fractured Kerboull-Postel plates, two were migrating cages, one was a medial wall mesh failure and one had been treated by impaction alone. In our series, results were disappointing where a large rim mesh or significant reconstruction was required. In light of these results, our technique has changed in that we now use predominantly larger chips of purely cancellous bone, 8-10 mm3 in size, to fill the cavity and larger diameter cups to better fill the aperture of the reconstructed acetabulum. In addition we now make greater use of i) implants made of a highly porous in-growth surface to constrain allograft chips and ii) bulk allografts combined with cages and morcellised chips in cases with very large segmental and cavitary defects.

Simulation of Fretting Wear at Orthopaedic Implant Interfaces

Osteolysis due to wear debris is a primary cause of failure of total joint replacements. Although debris produced by the joint articulating surfaces has been studied and simulated extensively, fretting wear debris, produced at nonarticulating surfaces, has not received adequate attention. We developed a three-station fretting wear simulator to reproduce in vivo motion and stresses at the interfaces of total joint replacements. The simulator is based on the beam bending theory and is capable of producing cyclic displacement from 3 to 1000 microns, under varying magnitudes of contact stresses. The simulator offers three potential advantages over previous studies: The ability to control the displacement by load, the ability to produce very small displacements, and dynamic normal loads as opposed to static. A pilot study was designed to test the functionality of the simulator, and verify that calculated displacements and loads produced the predicted differences between two commonly used porous ingrowth titanium alloy surfaces fretting against cortical bone. After 1.5 million cycles, the simulator functioned as designed, producing greater wear of bone against the rougher plasma-sprayed surface compared to the fiber-mesh surface, as predicted. A novel pin-on-disk apparatus for simulating fretting wear at orthopaedic implant interfaces due to micromotion is introduced. The test parameters measured with the fretting wear simulator were as predicted by design calculations, and were sufficient to measure differences in the height and weight of cortical bone pins rubbing against two porous ingrowth surfaces, plasma-sprayed titanium and titanium fiber mesh.

The prosthesis-bone interface adjacent to tibial components inserted without cement. Clinical and radiographic follow-up at nine to twelve years.

We evaluated the radiographic and functional status of a one-piece composite tibial component, designed to be inserted without cement, that was developed and used only at our institution. Thirty-six patients (thirty-seven knees) were managed with the component between November 1981 and September 1983, and none were lost to follow-up. When they were last seen, thirty-four (92 per cent) of the thirty-seven knees had a well-fixed tibial component, both clinically and radiographically. Eight patients (eight knees; 22 per cent) died. Eleven patients (eleven knees; 30 per cent) had a revision at an average of sixty-five months (range, four to ninety-five months) postoperatively. The reason for the revision was failure of a metal-backed patellar component in three knees, excessive wear of the polyethylene of the tibial component in one, a hematogenous infection in four, aseptic loosening of the tibial component in two, and chronic synovitis in one. The remaining seventeen patients (eighteen knees; 49 per cent) were seen for clinical and radiographic follow-up at an average of eleven years (range, nine to twelve years) after the operation. With removal of the implant as the end point, the cumulative rate of survival was 83 per cent at fifty-six months and 67 per cent at 108 months. Some of the failures were secondary to features of the prosthetic design that currently are considered to be inadequate, including a metal-backed patellar component and carbon-fiber-reinforced polyethylene. In the twenty-six knees in which the prosthesis had been retained, the implant was stable and the prosthesis-bone interface was unchanged as seen radiographically at the time of the most recent follow-up examination. This finding demonstrates that a porous ingrowth surface is capable of providing a secure interface for biological fixation over the long term.

Pain in the thigh following total hip replacement with a porous-coated anatomic prosthesis for osteoarthrosis. A five-year follow-up study.

We performed a study to determine the prevalence, severity, and natural history of pain in the thigh in patients who had a total hip replacement with a porous-coated anatomic prosthesis and to determine if there was an association between the radiographic findings and the pain in the thigh. We previously reported the results in these patients two years after the operation. The present study included ninety-four patients (101 total hip replacements) who had been followed for a minimum of five years--six of the original group of patients had died from unrelated causes, two had had a revision, and one had been lost to follow-up. The patients were assessed prospectively with use of the Harris hip score and a visual-analog scale for pain in the thigh. Two of us, who were blinded to the clinical result, reviewed the radiographs retrospectively for the morphology of the proximal aspect of the femur; distal fit; metaphyseal fill; alignment of the stem; shedding of beads; distal cortical hypertrophy; pedestal formation; cancellous condensations at the distal end of the porous ingrowth surface; and the presence, extent, and location of radiolucent lines around the femoral component. At five years, pain in the thigh was found in association with twenty-seven hips (27 per cent). The pain was new in fourteen thighs (14 per cent), unchanged (from that at the two-year follow-up examination) in eight (8 per cent), increased in four (4 per cent), and decreased in one (1 per cent). Pain in the thigh was most strongly associated with a poor Harris hip score and the presence of loose beads.

Artificial Disc Replacement

Artificial disc replacement was performed in six patients with an average age of 55 years and average follow-up of 3.4 years. Four of the six patients had juxtafusion degeneration, one had multilevel disc degeneration, and one patient had isolated disc resorption. The Acroflex disc (Acromed Corporation, Cleveland, OH), which was used in the replacement, is composed of a rubber core vulcanized to two titanium end plates. The latter have superior posts to provide for initial mechanical fixation and porous in-growth surfaces for long-term fixation. Satisfactory results occurred in four of six patients. Poor results occurred in the presence of deformity that resulted in prosthetic failure and isolated disc resorption. There was an average 8 degrees angular and 2.3-mm translational movement and satisfactory in-growth at all interfaces. Design objectives of endurance, biocompatibility, geometry, kinematics, constraint, dynamics, stability, and fail-safe were met; however, this study is preliminary in nature.

Effects of ingrowth, geometry, and material on stress transfer under porous-coated hip surface replacements.

Three different interface geometries for porous ingrowth surface replacements of the hip were examined using two-dimensional linear and nonlinear contact finite element analyses. The results indicate that incorporation of a nearly flat prosthesis interface between the surface replacement and the underlying cancellous bone may reduce stress shielding and improve stress transfer from the component. For all designs analyzed, the bone stress shielding was insensitive to component material stiffness when the elastic modulus was greater than 30 MPa. The use of titanium instead of cobalt-chrome (Co--Cr) as the prosthesis material therefore had a negligible effect on stress shielding.

Stress changes in the femoral head due to porous ingrowth surface replacement arthroplasty

Finite element stress analyses were conducted of the canine femoral head before and after implantation of various surface replacement-type components. The femoral head was replaced by four implant geometries; (a) shell, (b) shell with peg, (c) shell with rod, and (d) a new epiphyseal replacement design. All implants were modelled to simulate bony ingrowth along the underside of the shell and along the surfaces of the peg and rod. The results indicated that in the normal femur the forces are transferred from the articular surface through the femoral head cancellous bone to the inferior cortical shell of the femoral neck. After shell-type surface replacement, forces were transferred more distally at the rim of the shell and at the end of the peg or rod, thereby reducing the stresses in the proximal head cancellous bone. Computer simulation of bone remodelling due to proximal bone stress reduction was shown to accentuate the abnormality of the stress fields. Surface replacement with a lower modulus material created a less abnormal redistribution of bone stresses. The new epiphyseal replacement design resulted in stress distributions similar to those in the normal femoral head and minimal shear stresses at the implant/bone interface. These findings suggest that the epiphyseal replacement concept may provide better initial mechanical integrity and create a more benign milieu for adaptive bone remodelling than conventional, shell-type surface replacement components.