Powder metal-made orthopedic implants with porous surface for fixation by tissue ingrowth

Powder metal-made orthopedic implants with a porous coating provide an effective means for implant fixation by tissue ingrowth. Additionally, the use of metal alloy powders for forming porous surfaces offers the advantage of uniform coatings on complex part shapes. With proper processing, implants with strong, porous surface layers and good substrate mechanical properties can be formed. The in vivo tests have demonstrated the need for initial implant stability to achieve bony ingrowth. Animal studies indicate an optimum pore size range of 50-400 mu, and human hip prostheses with pores in this range appear to function well. A porous surface integrated with a compatible implant design avoids undesirable bone remodeling with these types of implants.     

Porous-coated femoral components in a canine model for revision arthroplasty

Six dogs had a total hip arthroplasty during which the femoral component was coated with methacrylate and inserted in the femoral canal, after the canal had been reamed to a larger diameter than that of the femoral stem (including the coating). Thus, the implant was loose and motion was present between it and the femur in each animal. Five dogs had a revision to a prosthesis with a porous polysulfone-coated stem. This prosthesis was not cemented in place. One dog was killed before the revision for the purpose of histological examination. Although bone was present in the porous surface of all five stems at the revision, the amount was scant in three. CLINICAL RELEVANCE: A model is described for the study of cemented hip-replacement prostheses that have failed. Although variable amounts of ingrowth of bone were observed after the revision to the porous-coated prostheses without bone cement, further study is needed to determine whether adequate ingrowth of bone occurs in this model.     

Composite fixation of salvage prostheses for the hip and knee.

A composite fixation method for a porous-coated modular segmental bone/joint prosthetic system was developed to improve fixation at revision surgery of the hip and knee after failure of the original implant with resulting massive bone loss. The porous coating is limited only to the segmental shoulder region of the prosthesis, while bone cement is used to provide initial implant stability. Autogenous bone grafts are applied over the porous-coated region to achieve extracortical bone bridging and ingrowth for long-term biologic fixation. Bench tests, theoretical analysis, and animal experiments were performed to validate this fixation concept. Clinical, roentgenographic, and functional results of revision patients using these prostheses and the underlying fixation principle are very encouraging. A number of limiting factors were identified and further improvements are being investigated to foster this concept as a viable salvage alternative.     

Histologic analysis of retrieved human porous-coated total joint components

A histologic and microradiographic analysis was performed on 90 retrieved human noncemented porous-coated total joint implants recovered from 58 patients. The specimens included 62 total knee components from 34 patients and 28 total hip components from 24 patients. All components were inserted without the use of bone cement, and in no case was the retrieved component removed due to clinically or roentgenographically apparent loosening. Approximately 92% of the total knee components and 93% of the total hip components had been in situ at least six weeks; 70% of the knee components and more than 50% of the hip components had been functional for at least nine months. The histologic sections and microradiographs revealed varying amounts of bone growth into or in apposition to the porous coatings. In approximately one third of the components, no bone ingrowth or apposition was observed. No component had greater than 10% of the available porous material ingrown with bone. No relationship between the degree of bone ingrowth and the length of time in situ was noted. In all components, the majority of the porous coating contained fibrous tissue that in some cases displayed orientation indicating evidence of load transmission capability. The adherence of bony tissue at the time of removal, a positive roentgenographic evaluation, or a positive clinical presentation was not found to be a definite prognosticator of bone ingrowth. It appears that the combination of limited bone ingrowth and extensive fibrous tissue ingrowth is adequate for implant fixation.     

Total knee arthroplasty. Two- to four-year experience using an asymmetric tibial tray and a deep trochlear-grooved femoral component

The design features of an implant system can be crucial to the success of cementless total knee arthroplasty (TKA). The implant system described in this study incorporates anatomic features including an asymmetric tibial tray that is smaller laterally than medially, and a deep trochlear-grooved femoral component. Two hundred consecutive primary total knees were implanted between October 18, 1985 and January 19, 1988 and were followed prospectively. One hundred ninety-two were implanted without cement. Clinical evaluation demonstrates good or excellent results in 96%, including a mean range of motion of 122 at two to four years after arthroplasty. Routine fluoroscopic spot roentgenograms have shown no complete radiolucency in this series of patients. Bone ingrowth was predictable if morselized autograft bone was interposed between implant and host bone. Implant retrievals demonstrated uniform bone ingrowth into the porous coating, increasing to 40% of the pore volume in the tibial tray at 27 months. Restoration of the normal kinematics appears to minimize the bone-implant stress permitting bone ingrowth fixation. The results of primary cementless TKA in this series is comparable to cemented TKA with the advantage of conserving bone stock.     

Porous-coated uncemented components in experimental total hip arthroplasty in dogs. Effect of plasma-sprayed calcium phosphate coatings on bone ingrowth.

The effect of a thin plasma-sprayed, calcium phosphate ceramic coating on bone ingrowth into titanium fiber mesh porous-surfaced prostheses was examined in a controlled canine cementless total hip arthroplasty model. Bone ingrowth was quantified using backscattered scanning electron microscopy of undemineralized sections. When good contact between the bone and porous coating was present, the calcium phosphate-coated prostheses contained significantly higher amounts of bone ingrowth at three weeks postimplantation than the uncoated control prostheses. At six weeks, however, there was no significant difference in the amount of bone ingrowth between the coated prostheses and uncoated prostheses. The ingrown bone seemed to be more intimately associated with the calcium phosphate-coated porous surfaces than the uncoated porous surfaces. When gaps at the bone-porous coating interface occurred, the calcium phosphate coating did not enhance bone ingrowth across those gaps. Plasma-sprayed calcium phosphate coatings may be useful in enhancing the early ingrowth of bone into porous-surfaced joint replacement prostheses, but they may lack long-term effectiveness. The coatings were not effective in enhancing bone ingrowth across gaps between the porous surface and the bone bed prepared at surgery.     

Bilateral tibial components of different cementless designs and materials. Microradiographic, backscattered imaging, and histologic analysis

Postmortem evaluation was conducted on two cementless knee prostheses considered clinically successful. The two retrieved uncemented porous-coated tibial components of different designs, and materials were evaluated by microradiography, backscattered electron (BSE) imaging, and light microscopy. The right prosthesis, in place for 25 months, was a Porous-Coated Anatomic (PCA) implant with double-layered, sintered, cobalt-chromium alloy beads. The left prosthesis was a Natural-Knee (N-K) implant with a porous coating of cancellous-structured pure titanium implanted for 19 months. A quantitative microradiographic index, the appositional bone index (ABI), was developed to indicate the probability of bone ingrowth occurring into the porous coating. The ABI is a ratio of bone in apposition with porous coating divided by the total amount of porous coating available. The PCA had an average ABI of 9%, and the N-K, 67%. BSE images of the PCA demonstrated no bone within the porous coating. BSE images of the N-K implant showed bone ingrowth into 22% of the pore volume when porous coating was in apposition to host bone. Histology of the PCA revealed fibrous connective tissue throughout the porous coating and between the porous coating-bone interface. Histology of the N-K implant revealed bone ingrowth and osteoblastic activity along the bone within the porous coating.     

Unicompartmental porous coated anatomic total knee arthroplasty

Unicompartmental arthroplasty has been advocated for management of the older patient with unicompartmental arthritis. Implant breakage and loosening has led to modifications of implant design to metal backing and porous coating for potential bone ingrowth. A review of 28 cementless porous coated anatomic total knee arthroplasties two years after the operation revealed good to excellent results in 20 knees (71%). Six knees in five patients required revision for persistent pain. Only fibrous tissue ingrowth was observed. Five knees were considered potential failures based upon persistent pain and roentgenographic evidence of component loosening. This implant design is not acceptable with cementless fixation.     

Macroscopic and microscopic evidence of prosthetic fixation with porous-coated materials

The host response to porous-coated prostheses appears favorable; there is little evidence of any adverse tissue response or significant osteoclastic activity except in grossly loose specimens. While the nature of retrieval specimens makes any statistical correlation problematic, some generalizations can be made. Femoral hip prostheses are most likely to present bone ingrowth along the lateral quadrant of their porous coating. The frequency of bone ingrowth of femoral components was nearly twice that of acetabular devices. Pore size, geometry, and porous-coating composition did not appear to influence the appearance of bone and fibrous tissue ingrowth. Direct bonding of bone to the uncoated portion of the prosthesis was rarely seen and occurred only in closest proximity to the porous-coated regions. Indications of pain and looseness are evidence that fibrous tissue ingrowth alone is not always sufficient to ensure stability. Additionally, some bone-ingrown prostheses were retrieved because of pain, which leads to the conclusion that local bone ingrowth cannot ensure a general freedom from pain, especially with partially coated prostheses. Bone and fibrous tissue response to the porous coatings generally consists of interdigitation, while the response to uncoated regions is fibrous tissue encapsulation. Burnishing the distal tips of many of the partially coated femoral prostheses is an indication of relative motion in that region, which may be a potential source of pain.     

Proximal femoral osteotomy as an adjunct in cementless revision total hip arthroplasty

Cementless total hip replacement techniques are increasingly used in revision arthroplasty. A major challenge is to achieve implant stability in a femur distorted by a failed arthroplasty. Five patients with aseptic loosening of cemented primary or revision total hip replacements complicated by significant proximal femoral disease, four with marked angular deformity of the femur and one with a subtrochanteric nonunion, were treated successfully with cementless revision arthroplasty combined with proximal femoral osteotomy. At follow-up examination, all femoral and trochanteric osteotomies had healed and D'Aubigne and Postel scores for pain, function, and range of motion had improved. All porous prostheses demonstrated radiographic features consistent with bone ingrowth fixation. No progressive stress shielding has been observed. Concomitant femoral osteotomy to correct anatomic deformity, in association with cementless total hip arthroplasty, results in union of the osteotomy and restoration of hip function.     

Results of noncemented revision

The results of revision of total hip replacements with noncemented fixation are dependent on implant design, postoperative treatment, and the technique selected in relationship to the bone quality and quantity. When proximal femoral bone ingrowth fixation is desired an implant must have a double wedge design, which permits a high percentage contact between the host bone and the porous coating of the implant. Bone graft in either the femur or acetabulum should be used at a minimum. Every attempt to obtain host bone contact to porous coating should be done. When a bone graft is used to replace the proximal femur or when an acetabular bone graft covers more than 50% of the cup, the implant should be cemented into the bone graft. Bone structure affects the outcome of noncemented revisions more than the thickness of the bone. Even thin and weak femoral bone has dynamic remodeling capability. The structure influences result more according to the ability to contact the host bone to the porous coating. When this can be done, the bone will heal to the porous coating. Postoperatively, 40% of our patients are treated with a cast to protect bone grafts and weak bones that are important for stability of the implant. We believe that conservative postoperative rehabilitation for noncemented revision arthroplasty significantly improves the healing for the bone to the implant.     

Bone ingrowth observed in a cup removed during revision surgery for early dislocation after primary THA: A case report

Introduction and importanceRecently cementless total hip arthroplasty (THA) has shown good long-term results with excellent stability resulted from the porous coating of the implant. A hydroxyapatite-tricalcium phosphate (HA-TCP) coating on the porous surface is expected to promote bone ingrowth and to improve initial fixation of the implant. Here we report a case of bone ingrowth observed in a cup removed during revision surgery for early dislocation, 37 days after primary THA using a porous coating cup with HA-TCP.Case presentationA 61-year-old woman who has bilateral osteoarthritis underwent same-day bilateral THA. Both sides used porous coating cups with HA-TCP. Line-to-line technique and screw fixation were utilized. Anterior dislocation of the left hip occurred on days 27 and 31, and we performed cup revision on day 37 after surgery. We noted bone-like tissue on the posterior surface of the cup and in a vacant screw hole. Bone tissue was also confirmed in pathological findings.Clinical discussionStudies in animals have confirmed early bone ingrowth about 4 weeks after surgery with HA-TCP coated implants. In humans, the earliest report of bone ingrowth in cups is for a cup without HA-TCP coating, detected 5 weeks after surgery. In the present case, we used a porous coating cup with HA-TCP, and bone ingrowth was confirmed at approximately the same time as for the previous case.ConclusionEarly clinical bone ingrowth was confirmed in an HA-TCP coated cup, occurring at about the same time after surgery as in previous reports.     

Hydroxyapatite-coated total hip prostheses. Two-year clinical and roentgenographic results of 100 cases

Studies of implant fixation have shown that hydroxyapatite (HA) coatings provide early and strong fixation to bone. This is a report of 100 consecutive cases of total hip arthroplasties, using HA coating, which were mainly for osteoarthrosis, avascular necrosis, or revision for failed implants. The average prospective follow-up period was two years. Titanium femoral components had a proximal HA coating, usually with an HA-coated screw cup. For both HA-coated components, the average Harris hip score was 96 points after one year and 98 after two years. Analysis of data shows that the incidence of pain was low immediately after surgery and at 4% one year postoperatively. There was no difference between the results of primary cases and revisions after the one-year interval. On roentgenographic examination, there was a rapid bony integration of implants with bone apposition on the coating within six months, accompanied by specific patterns of remodeling. No radiolucent line formation was detected around HA-coated implant parts. There were no revisions for loose implants. After two years, 97% of the patients had positive roentgenographic evidence of femoral ingrowth compared to 55% for HA-coated acetabular cups, with a statistical significance between bone ingrowth and clinical results. Implant fixation using HA coatings is a reliable procedure for good bony fixation and clinical results.     

The influence of stem size and extent of porous coating on femoral bone resorption after primary cementless hip arthroplasty

The influence of stem size and extent of porous coating on femoral bone resorption was examined in 411 cases of primary cementless hip arthroplasty. Moore design, cobalt alloy femoral implants with powder-made sintered porous coating on either one-third, two-thirds, or the full implant length were compared radiographically two years after surgery. A semiquantitative method was adopted for assessing resorption that involved dividing the anteroposterior (AP) and lateral roentgenograms into a total of 16 discrete sites. The 16 sites were qualitatively examined for evidence of resorption by either thinning or darkening of bone relative to the time immediately following surgery. Based on the number of sites that demonstrated resorption, the bone loss was classified as either minor and not likely to cause problems (0 to 4 sites) or pronounced and of potentially harmful clinical consequence (5 or more sites). Pronounced resorption occurred in 18% of the 411 cases. The use of larger stems resulted in increased occurrence of marked bone resorption: stems greater than or equal to 13.5 mm in diameter showed five times the incidence of pronounced resorption compared with stems less than or equal to 12.0 mm in diameter. Stems with two-thirds and full porous coating resulted in a twofold to fourfold increase in the incidence of pronounced bone resorption. The theoretic degree of stress shielding of the femoral shaft in bending was calculated for cases with complete canal filling and a radiographic appearance of bone ingrowth. There was a strong correlation between this theoretic factor and the observed bone resorption.     

Total hip replacement with cemented, uncemented, and hybrid prostheses. A comparison of clinical and radiographic results at two to four years

One hundred and thirty-one patients who had 144 cemented or uncemented hip prostheses were followed prospectively for two to four years. A cemented or a hybrid prosthesis (consisting of a cemented femoral component and an uncemented acetabular component) was used in men older than seventy years, in women older than sixty years, and in younger patients in whom adequate initial fixation could not be obtained without cement. Uncemented, porous-surfaced implants were used in all other patients. The over-all clinical results were similar for the three groups. For the fifty-two hips that had a cemented prosthesis, the mean total Harris hip rating was 91 points and the score for pain, 42 points; for the twenty-seven hips that had a hybrid prosthesis, 90 and 43 points; and for the sixty-five hips that had an implant allowing ingrowth of bone in both the acetabulum and the femur, 95 and 43 points. Two prosthetic stems that were designed to allow ingrowth of bone had aseptic loosening; one was revised. Pain in the thigh, usually slight and not disabling, occurred at one year in 24 per cent of the patients in whom a femoral component allowing ingrowth had been used; the prevalence of pain then declined. The incidences of migration of the components and of radiolucent lines were greater in the acetabula that had a cemented component than in those that had a cup allowing ingrowth of bone.     

Histologic analysis of a retrieved microporous-coated femoral prosthesis. A seven-year case report

A microporous-coated femoral prosthesis with an average pore size of 80 to 100 microns was retrieved after seven years in a revision operation on a failed cemented acetabular component. The proximal anterior and lateral aspects of the implant were encrusted with bone. The bone was attached to the porous coating in regions of ingrowth. The implant was stable within the femur although most of the porous coating was not bone ingrown. There was progressive bone resorption in the proximal femur, suggestive of a reactive biologic mechanism. There was also a sizable posterior pocket of granulation tissue containing chronic inflammatory round cells associated with numerous birefringent polymeric particles. Polyethylene wear debris and acrylic cement were engulfed by multinucleated giant cells enveloping the loose cup.     

Quantification of implant micromotion, strain shielding, and bone resorption with porous-coated anatomic medullary locking femoral prostheses.

Fourteen femora containing porous-coated anatomic medullary locking (AML) femoral prostheses were retrieved from 12 patients at autopsy. Clinical roentgenograms in 13 femora showed bone remodeling changes, indicating that the implants were fixed by osseointegration. Under simulated physiologic loading, micromotion between the implant and the bone was measured using electrical displacement transducers connected to the implant and to the adjacent cortex. The micromotion between the implants at the areas of porous coating and the adjacent cortex in the one case of failed bone ingrowth measured 150 microns. Maximum relative motion between the cortex and the implant in the areas of porous coating for the 13 cases showing signs of bone ingrowth was 40 microns, and this was completely elastic relative displacement. With all implants, the micromotion between the cortex and the stem was always greatest over the uncoated portion of the stem. Four of the implants were proximally porous coated. With these, the micromotion was greater over the uncoated areas than with more extensively coated stems and was always greatest at the uncoated tip of the prosthesis. The amount of micromotion was directly related to the extent of porous coating on the implant. Maximum tip motion for the proximally coated implants was 210 micra, whereas for the fully porous-coated implants, it was 40 microns. In nine of the autopsies, the contralateral normal femur was obtained in addition to the femur containing the AML (the in vivo remodeled femur). These were used for comparative studies of strain shielding and femoral remodeling. Cortical strains were measured in the in vivo remodeled femora and were compared with measurements made in the contralateral normal femora before and following implantation of a stem identical to that present on the clinically treated side. The data showed major strain reductions in all the postmortem implanted normal femora. Comparison of the strain data from the postmortem implanted normal femora with those from the in vivo remodeled femora clearly indicated that extensive bone remodeling did not result in restoration of cortical strain levels anywhere near normal. Strain shielding continued to exist in all of the remodeled specimens, even up to 7.5 years after surgery. This strain shielding was associated with bone remodeling changes that resulted in regional reductions in bone mineral content that ranged from 7% to 78%. These observations are unique, important, and valuable in defining the in vivo function and clinical behavior of this type of porous-coated femoral component.     

A comparison of bone remodelling around hydroxyapatite-coated, porous-coated and grit-blasted hip replacements retrieved at post-mortem

We investigated the implant-bone interface around one design of femoral stem, proximally coated with either a plasma-sprayed porous coating (plain porous) or a hydroxyapatite porous coating (porous HA), or which had been grit-blasted (Interlok). Of 165 patients implanted with a Bimetric hip hemiarthroplasty (Biomet, Bridgend, UK) specimens were retrieved from 58 at post-mortem. We estimated ingrowth and attachment of bone to the surface of the implant in 21 of these, eight plain porous, seven porous HA and six Interlok, using image analysis and light morphometric techniques. The amount of HA coating was also quantified. There was significantly more ingrowth (p = 0.012) and attachment of bone (p < 0.05) to the porous HA surface (mean bone ingrowth 29.093 +/- 2.019%; mean bone attachment 37.287 +/- 2.489%) than to the plain porous surface (mean bone ingrowth 21.762 +/- 2.068%; mean bone attachment 18.9411 +/- 1.971%). There was no significant difference in attachment between the plain porous and Interlok surfaces. Bone grew more evenly over the surface of the HA coating whereas on the porous surface, bone ingrowth and attachment occurred more on the distal and medial parts of the coated surface. No significant differences in the volume of HA were found with the passage of time. This study shows that HA coating increases the amount of ingrowth and attachment of bone and leads to a more even distribution of bone over the surface of the implant. This may have implications in reducing stress shielding and limiting osteolysis induced by wear particles.     

Current knowledge and future perspectives of cementless endoprosthetics

The current problems facing the clinician, regarding young patients whose hips and knees have completely deteriorated, involve a return to function, as well as a longer life for the prosthesis than that rendered by cement fixation. The alternatives to cemented primary fixation devices have not yet with stood the test of time. In addition, when aseptic loosening of cemented prostheses occurs, cemented revisions have already demonstrated a high failure rate after a short-term follow-up, particularly in the hip. A follow-up period of up to 6 years for a cementless total knee and a short-term follow-up for a cementless hip (up to 3 years), using a porous metal bone contact surface, have shown very satisfactory clinical results in primary total hip and knee prostheses and revision of the total hip without the use of bone cement. These early results justify continuation of this clinical trial.     

Prospective evaluation of total hip arthroplasty with a cementless, anatomically designed, porous-coated femoral implant: mean 11-year follow-up

The performance of an anatomically designed femoral implant with porous titanium mesh coating was assessed in patients undergoing total hip arthroplasty. The stem, with built-in 12 degrees anteversion, was implanted in neutral position relative to the long axis of the femur through a cementless press-fit application. The only requirement was the presence of sufficient thickness within the cortical bone of the femoral metaphysis to support the stem. Ninety-one evaluable patients were followed up for a minimum of 10 years. Patients tolerated weight-bearing activity within 24 hours after surgery. Only 1 femoral component required revision because of loosening. Most patients showed uniform bony ingrowth. Quality of life was markedly improved. The anatomically designed, porous-coated implant provides robust performance which has general utility in a wide range of patients.