Outcomes of total hip and knee replacement: preoperative functional status predicts outcomes at six months after surgery

OBJECTIVE

To determine whether patients with knee or hip osteoarthritis (OA) who have worse physical function preoperatively achieve a postoperative status that is similar to that of patients with better preoperative function.

METHODS

This study surveyed an observational cohort of 379 consecutive patients with definite OA who were without other inflammatory joint diseases and were undergoing either total hip or knee replacement in a US (Boston) and a Canadian (Montreal) referral center. Questionnaires on health status (the Short Form 36 and Western Ontario and McMaster Universities Osteoarthritis Index) were administered preoperatively and at 3 and 6 months postoperatively. Physical function and pain due to OA were deemed the most significant outcomes to study.

RESULTS

Two hundred twenty-two patients returned their questionnaires. Patients in the 2 centers were comparable in age, sex, time to surgery, and proportion of hip/knee surgery. The Boston group had more education, lower comorbidity, and more cemented knee prostheses. Patients undergoing hip or knee replacement in Montreal had lower preoperative physical function and more pain than their Boston counterparts. In patients with lower preoperative physical function, function and pain were not improved postoperatively to the level achieved by those with higher preoperative function. This was most striking in patients undergoing total knee replacement.

CONCLUSION

Surgery performed later in the natural history of functional decline due to OA of the knee, and possibly of the hip, results in worse postoperative functional status.

Characteristics of bone ingrowth and interface mechanics of a new porous tantalum biomaterial

We have studied the characteristics of bone ingrowth of a new porous tantalum biomaterial in a simple transcortical canine model using cylindrical implants 5 x 10 mm in size. The material was 75% to 80% porous by volume and had a repeating arrangement of slender interconnecting struts which formed a regular array of dodecahedron-shaped pores. We performed histological studies on two types of material, one with a smaller pore size averaging 430 microm at 4, 16 and 52 weeks and the other with a larger pore size averaging 650 microm at 2, 3, 4, 16 and 52 weeks. Mechanical push-out tests at 4 and 16 weeks were used to assess the shear strength of the bone-implant interface on implants of the smaller pore size. The extent of filling of the pores of the tantalum material with new bone increased from 13% at two weeks to between 42% and 53% at four weeks. By 16 and 52 weeks the average extent of bone ingrowth ranged from 63% to 80%. The tissue response to the small and large pore sizes was similar, with regions of contact between bone and implant increasing with time and with evidence of Haversian remodelling within the pores at later periods. Mechanical tests at four weeks indicated a minimum shear fixation strength of 18.5 MPa, substantially higher than has been obtained with other porous materials with less volumetric porosity. This porous tantalum biomaterial has desirable characteristics for bone ingrowth; further studies are warranted to ascertain its potential for clinical reconstructive orthopaedics.

Evidence for altered synthesis of type II collagen in patients with osteoarthritis

There is evidence to suggest that the synthesis of type II collagen is increased in osteoarthritis (OA). Using an immunoassay, we show that the content of the C-propeptide of type II procollagen (CPII), released extracellularly from the newly synthesized molecule, is directly related to the synthesis of this molecule in healthy and osteoarthritic articular cartilages. In OA cartilage, CPII content is often markedly elevated (mean 7.6-fold), particularly in the mid and deep zones, reaching 29.6% of the content in newborn. Synthesis is also directly related to total collagen II content in OA, suggesting its importance in maintaining collagen content and cartilage structure. The release of CPII from cartilage is correlated directly with cartilage content. However, the increase in CPII in OA cartilage is not reflected in serum, where a significant reduction is observed. Together these studies provide evidence for alterations in procollagen II synthesis in vivo in patients with OA.

Tissue response to porous tantalum acetabular cups: a canine model

This study evaluated the osseous tissue response to a noncemented metal-backed acetabular component made of a new porous tantalum biomaterial. Eleven dogs with bilateral total hip arthroplasties (22 acetabular implants) were studied for a period of 6 months. Thin section histology, high-resolution radiography, and backscattered scanning electron microscopy revealed that all 22 implants had stable bone-implant interfaces. Regions of bone ingrowth were present in all histologic sections. The depth of bone ingrowth varied from 0.2 mm to the maximal limit of 2 mm. Analyzing contiguous regions of interest across the full bone-implant interface, the mean bone ingrowth for all sections was 16.8% +/- 5.7%. In the peripheral regions of the cup where bone-implant contact was most consistent, bone ingrowth averaged 25.1% +/- 10.1%. The data indicate that the porous tantalum material is effective for biologic fixation in the dog and may provide a suitable alternative to other porous materials used in acetabular cup design.

The Otto Aufranc Award. Wear and lubrication of metal-on-metal hip implants

The implication of polyethylene wear particles as the dominant cause of periprosthetic osteolysis has created a resurgence of interest in metal-on-metal implants for total hip arthroplasty because of their potential for improved wear performance. Twenty-two cobalt chromium molybdenum metal-on-metal implants were custom-manufactured and tested in a hip simulator. Accelerated wear occurred within the first million cycles followed by a marked decrease in wear rate to low steady-state values. The volumetric wear at 3 million cycles was very small, ranging from 0.15 to 2.56 mm3 for all implants tested. Larger head-cup clearance and increased surface roughness were associated with increased wear. Independent effects on wear of material processing (wrought, cast) and carbon content were not identified. Implant wear decreased with increasing lambda ratio, a parameter used to relate lubricant film thickness to surface roughness, suggesting some degree of fluid film lubrication during testing. This study provided important insight into the design and engineering parameters that affect the wear behavior of metal-on-metal hip implants and indicated that high quality manufacturing can reproducibly lead to very low wear.

Selective enhancement of collagenase-mediated cleavage of resident type II collagen in cultured osteoarthritic cartilage and arrest with a synthetic inhibitor that spares collagenase 1 (matrix metalloproteinase 1)

OBJECTIVE

To examine whether type II collagen cleavage by collagenase and loss of proteoglycan are excessive in human osteoarthritic (OA) articular cartilage compared with nonarthritic articular cartilage, and whether this can be inhibited by a selective synthetic inhibitor that spares collagenase 1 (matrix metalloproteinase 1 [MMP-1]).

METHODS

Articular cartilage samples were obtained during surgery from 11 patients with OA and at autopsy from 5 adults without arthritis. The articular cartilage samples were cultured in serum-free medium. A collagenase-generated neoepitope, which reflects cleavage of type II collagen, and proteoglycan glycosaminoglycan (GAG), which predominantly reflects aggrecan release, were assayed in culture media. In addition, cultures were performed using either of 2 synthetic MMP inhibitors, both of which inhibited collagenase 2 (MMP-8) and collagenase 3 (MMP-13), but one of which spared collagenase 1. Cultures were also biolabeled with 3H-proline in the presence and absence of these inhibitors to measure collagen synthesis (as tritiated hydroxyproline) and incorporation in articular cartilage.

RESULTS

As a group, cleavage of type II collagen by collagenase was significantly increased in OA cartilage samples. In contrast, proteoglycan (GAG) release was not increased. This release of a collagenase-generated epitope was inhibited by both MMP inhibitors in 2 of 5 nonarthritic samples and in 9 of 11 OA cartilage samples. The inhibitor that spared collagenase 1 was generally more effective and inhibited release from 4 of 5 nonarthritic cartilage samples and the same OA cartilage samples. Group analyses revealed that the inhibition of collagenase neoepitope release by both inhibitors was significant in the OA patient cartilage, but not in the nonarthritic cartilage. Proteoglycan loss was unaffected by either inhibitor. Newly synthesized collagen (predominantly, type II) exhibited increased incorporation in OA cartilage, but only in the presence of the inhibitor that arrested collagenase 1 activity.

CONCLUSION

These results further indicate that the digestion of type II collagen by collagenase is selectively increased in OA cartilage, and that this can be inhibited in the majority of cases by a synthetic inhibitor that can inhibit collagenases 2 and 3, but not collagenase 1. The results also suggest that in OA, newly synthesized collagen is digested, but in a different manner than that of resident molecules. Proteoglycan release was not increased in OA cartilage and was unaffected by these inhibitors. Inhibitors of this kind may be of value in preventing damage to type II collagen in human arthritic articular cartilage.

Fibrous tissue ingrowth and attachment to porous tantalum

This study determined the soft tissue attachment strength and extent of ingrowth to a porous tantalum biomaterial. Eight dorsal subcutaneous implants (in two dogs) were evaluated at 4, 8, and 16 weeks. Upon retrieval, all implants were surrounded completely by adherent soft tissue. Implants were harvested with a tissue flap on the cutaneous aspect and peel tested in a servo-hydraulic tensile test machine at a rate of 5 mm/min. Following testing, implants were dehydrated in a solution of basic fuschin, defatted, embedded in methylmethacrylate, and processed for thin-section histology. At 4, 8, and 16 weeks, the attachment strength to porous tantalum was 61, 71, and 89 g/mm respectively. Histologic analysis showed complete tissue ingrowth throughout the porous tantalum implant. Blood vessels were visible at the interface of and within the porous tantalum material. Tissue maturity and vascularity increased with time. The tissue attachment strength to porous tantalum was three- to six-fold greater than was reported in a similar study with porous beads. This study demonstrated that porous tantalum permits rapid ingrowth of vascularized soft tissue, and attains soft tissue attachment strengths greater than with porous beads.

New femoral designs: do they influence stress shielding?

Virtually all contemporary cementless femoral hip stems are designed with the goals of achieving immediate and long-term stability, restoring hip mechanics, and minimizing thigh pain. However, the incorporation of features specifically intended to minimize stress-mediated bone resorption (stress shielding) has been variable. Attempts to reduce bone loss through stem design have yielded inconsistent results and, in certain instances, early and catastrophic failure. Prior beliefs regarding the determinants of stress shielding were based upon the qualitative assessment of bone loss using plain radiographs. These are being challenged, particularly with regard to the role of porous coating level. This is in large part due to the refinement and widespread availability of dual-energy xray absorptiometry (DEXA), which allows quantitative assessment of bone mineral density both pre- and postoperatively. The available evidence indicates stem stiffness plays a dominant role. Progressive bone loss through stress shielding has potentially dire consequences. While such problems have not manifested as severe or widespread clinical issues, the preservation of femoral bone stock is an important and desirable goal.

Evolutionary divergence of the necroptosis effector MLKL

The pseudokinase, MLKL (mixed-lineage kinase domain-like), is the most terminal obligatory component of the necroptosis cell death pathway known. Phosphorylation of the MLKL pseudokinase domain by the protein kinase, receptor interacting protein kinase-3 (RIPK3), is known to be the key step in MLKL activation. This phosphorylation event is believed to trigger a molecular switch, leading to exposure of the N-terminal four-helix bundle (4HB) domain of MLKL, its oligomerization, membrane translocation and ultimately cell death. To examine how well this process is evolutionarily conserved, we analysed the function of MLKL orthologues. Surprisingly, and unlike their mouse, horse and frog counterparts, human, chicken and stickleback 4HB domains were unable to induce cell death when expressed in murine fibroblasts. Forced dimerization of the human MLKL 4HB domain overcame this defect and triggered cell death in human and mouse cell lines. Furthermore, recombinant proteins from mouse, frog, human and chicken MLKL, all of which contained a 4HB domain, permeabilized liposomes, and were most effective on those designed to mimic plasma membrane composition. These studies demonstrate that the membrane-permeabilization function of the 4HB domain is evolutionarily conserved, but reveal that execution of necroptotic death by it relies on additional factors that are poorly conserved even among closely related species.

Relative contributions of chemistry and topography to the osseointegration of hydroxyapatite coatings

The purpose of the current study was to ascertain the relative contributions of surface chemistry and topography to the osseointegration of hydroxyapatite-coated implants. A canine femoral intramedullary implant model was used to compare the osseous response to commercially pure titanium implants that were either polished, grit-blasted, plasma-sprayed with hydroxyapatite, or plasma-sprayed with hydroxyapatite and masked with a very thin layer of titanium using physical vapor deposition (titanium mask). The titanium mask isolated the chemistry of the underlying hydroxyapatite layer without functionally changing its surface topography and morphologic features. At 12 weeks, the bone-implant specimens were prepared for undecalcified thin section histologic evaluation and serial transverse sections were quantified with backscattered scanning electron microscopy for the percentage of bone apposition to the implant surface. Bone apposition averaged 3% for the polished implants and 23% for the grit-blasted implants. Bone apposition to the hydroxyapatite-coated implants averaged 74% whereas bone apposition to the titanium mask implants averaged 59%. Although there was significantly greater osseointegration with the hydroxyapatite-coated implants, 80% of the maximum bone forming response to the implant surfaces developed with the titanium mask implants. This simple, controlled experiment revealed that topography is the dominant factor governing bone apposition to hydroxyapatite-coated implants.

Femoral remodeling after porous-coated total hip arthroplasty with and without hydroxyapatite-tricalcium phosphate coating: a prospective randomized trial

We prospectively assessed femoral bone remodeling using dual-energy x-ray absorptiometry for 2 years after total hip arthroplasty. Thirty-nine hips were randomized to receive a titanium proximally porous-coated femoral component with or without hydroxyapatite-tricalcium phosphate coating. Although both stems resulted in alterations in the periprosthetic bone mineral density, the hydroxyapatite-tricalcium phosphate coated stems had significantly less femoral bone loss than the uncoated stems at 2-year follow-up. This reduced femoral bone loss may provide short-term and long-term advantages over noncoated stems.

The relationship of intercondylar notch size and content to notchplasty requirement in anterior cruciate ligament surgery

A stenotic intercondylar notch is an indication for a notchplasty at the time of ACL reconstructive surgery. However, the absence of a stenotic notch does not ensure that impingement of the ligament upon the intercondylar notch will not occur. An excessively large ligament, even in the presence of a normal intercondylar notch, will result in abrasion of the ligament on the notch. This can result in the premature failure of the reconstructed ACL. Therefore, both the notch size and notch content are important determinants in assessing whether a notchplasty is required as part of the ACL reconstructive procedure.

Mechanical characterization of structurally porous biomaterials built via additive manufacturing: experiments, predictive models, and design maps for load-bearing bone replacement implants

Porous biomaterials can be additively manufactured with micro-architecture tailored to satisfy the stringent mechano-biological requirements imposed by bone replacement implants. In a previous investigation, we introduced structurally porous biomaterials, featuring strength five times stronger than commercially available porous materials, and confirmed their bone ingrowth capability in an in vivo canine model. While encouraging, the manufactured biomaterials showed geometric mismatches between their internal porous architecture and that of its as-designed counterpart, as well as discrepancies between predicted and tested mechanical properties, issues not fully elucidated. In this work, we propose a systematic approach integrating computed tomography, mechanical testing, and statistical analysis of geometric imperfections to generate statistical based numerical models of high-strength additively manufactured porous biomaterials. The method is used to develop morphology and mechanical maps that illustrate the role played by pore size, porosity, strut thickness, and topology on the relations governing their elastic modulus and compressive yield strength. Overall, there are mismatches between the mechanical properties of ideal-geometry models and as-manufactured porous biomaterials with average errors of 49% and 41% respectively for compressive elastic modulus and yield strength. The proposed methodology gives more accurate predictions for the compressive stiffness and the compressive strength properties with a reduction of the average error to 11% and 7.6%. The implications of the results and the methodology here introduced are discussed in the relevant biomechanical and clinical context, with insight that highlights promises and limitations of additively manufactured porous biomaterials for load-bearing bone replacement implants.

STATEMENT OF SIGNIFICANCE

In this work, we perform mechanical characterization of load-bearing porous biomaterials for bone replacement over their entire design space. Results capture the shift in geometry and mechanical properties between as-designed and as-manufactured biomaterials induced by additive manufacturing. Characterization of this shift is crucial to ensure appropriate manufacturing of bone replacement implants that enable biological fixation through bone ingrowth as well as mechanical property harmonization with the native bone tissue. In addition, we propose a method to include manufacturing imperfections in the numerical models that can reduce the discrepancy between predicted and tested properties. The results give insight into the use of structurally porous biomaterials for the design and additive fabrication of load-bearing implants for bone replacement.

Histopathologic retrieval analysis of clinically failed porous tantalum osteonecrosis implants

BACKGROUND

An Osteonecrosis Intervention Implant made of porous tantalum was recently developed to provide structural support and a compatible surface for tissue ingrowth in osteonecrotic femoral heads. From an investigational device exemption study that comprised 113 implants, we carried out a retrieval analysis of clinically failed implants.

METHODS

Seventeen porous tantalum implants that had been used for the treatment of Steinberg stage-II osteonecrosis of the hip were retrieved at the time of conversion to a total hip arthroplasty. Fifteen implants that had been transected near the base of the femoral neck with the proximal portion left in situ within the femoral head underwent histopathologic analysis at an average of 13.4 months (range, three to thirty-six months) after implantation.

RESULTS

Residual osteonecrosis was present in fourteen of the fifteen specimens. Fracture of the subchondral bone of the femoral head was present in all instances, and collapse of the femoral head was present in nine instances (60%). Backscattered scanning electron microscopy confirmed the presence of bone ingrowth in thirteen (87%) of the fifteen specimens. The mean extent of bone ingrowth was 1.9% (range, 0% to 4.4%).

CONCLUSIONS

The retrieved implants were associated with little bone ingrowth and insufficient mechanical support of subchondral bone. The implant design, the surgical technique, its application, and the clinical characteristics of candidates for this procedure should continue to be monitored closely.

Effect of noninvasive low intensity ultrasound on bone growth into porous-coated implants

Noninvasive low intensity ultrasound has been shown to be an effective means of accelerating bone fracture healing in both animal and clinical studies. An in vivo canine study was designed to determine if noninvasive low intensity ultrasound could influence the rate and extent of bone growth into porous-coated implants. Twenty-two pairs of fully porous transcortical implants were inserted bilaterally into the femora of 12 dogs. In each dog, one femur served as a control and the other was subjected to daily ultrasound stimulation for 2, 3, or 4 weeks. Overall, the ultrasound-stimulated implants demonstrated an 18% increase in bone ingrowth compared with their contralateral controls (p = 0.02). Noninvasive low intensity ultrasound had its greatest effect in the first 2-3 weeks of stimulation. At 2 and 3 weeks, the ultrasound-stimulated implants showed 21 and 16% more ingrowth than their respective contralateral controls. Because noninvasive low intensity ultrasound had a positive effect on bone ingrowth in this experimental investigation, further research is suggested to assess the clinical potential for application to noncemented porous-coated total joint replacements.

Zoledronic acid causes enhancement of bone growth into porous implants

The effect of zoledronic acid on bone ingrowth was examined in an animal model in which porous tantalum implants were placed bilaterally within the ulnae of seven dogs. Zoledronic acid in saline was administered via a single post-operative intravenous injection at a dose of 0.1 mg/kg. The ulnae were harvested six weeks after surgery. Undecalcified transverse histological sections of the implant-bone interfaces were imaged with backscattered scanning electron microscopy and the percentage of available pore space that was filled with new bone was calculated. The mean extent of bone ingrowth was 6.6% for the control implants and 12.2% for the zoledronic acid-treated implants, an absolute difference of 5.6% (95% confidence interval, 1.2 to 10.1) and a relative difference of 85% which was statistically significant. Individual islands of new bone formation within the implant pores were similar in number in both groups but were 69% larger in the zoledronic acid-treated group. The bisphosphonate zoledronic acid should be further investigated for use in accelerating or enhancing the biological fixation of implants to bone.

Potential errors in axial alignment using intramedullary instrumentation for total knee arthroplasty

Accurate restoration of normal limb alignment is crucial for the long-term survivorship of total knee arthroplasty (TKA). A mathematical model was used to evaluate the maximum error in varus and valgus alignment that could occur when cutting the tibia or femur during TKA using intramedullary (IM) guides of varying length and diameter. Minor deviations in the insertion point of IM instrumentation during TKA can result in malalignment of several degrees. This error can be minimized by careful attention to the entry point of the IM instrumentation or by increasing the IM rod diameter and length used during primary TKA.

In vitro measurement of the tracking pattern of the human patella

This study sought to determine whether a general pattern describing the three-dimensional tracking characteristics of the human patella could be established, and if not, then to determine the extent and nature of interspecimen variations in the characteristics in a normal population. Using 32 fresh-frozen knees subjected to extensor moment magnitudes similar to those in "static-lifting" and "leg-raising against resistance" maneuvers, patellar displacements were measured in the knee extension range 120 to 0 deg. For static-lifting, a constant foot-floor reaction of 334 N was applied. For leg-raising, a constant net quadriceps tension of 668 N was used throughout the extension range. Measurements were taken with a calibrated six-degree-of-freedom electromechanical goniometer and a displacement coordinate system referenced to the geometry of individual specimens. The three patellar displacements in the plane of knee extension/flexion (extension rotation, and anterior and proximal translations) consistently demonstrated the same pattern in the entire knee extension range with an average coefficient of variation of 13 percent. For knee angles greater than 45 deg, the three other displacements (medial-lateral translation, and rotations about the anterior--posterior and proximal--distal axes) followed a general pattern. However, for knee angles less than 45 deg, these displacements differed considerably between specimens for each loading condition, both in terms of magnitude (average coefficient of variation: 70 percent), and direction.

Bone densitometry: influence of prosthetic design and hydroxyapatite coating on regional adaptive bone remodelling

The objective of this prospective study was to determine if bone densitometry can detect disparities in regional adaptive bone remodelling surrounding the cementless porous-coated femoral component of a hip prosthesis in two titanium alloy implants of different design . These prostheses were the S-ROM (n=69) and the Multilock (n=65). The Multilock implants consisted of two groups; 25 had a 50 micron layer of hydroxyapatite (HA) sprayed over the porous surface of the femoral component and the remaining 40 femoral components were not coated with HA. Densitometry was performed with dual energy X-ray absorptiometry (DXA) utilizing the LUNAR ORTHO software to analyse the seven Gruen zones. Bone mineral density measurements were obtained within a week of surgery as a baseline reference and at 6, 12, 24, 36 and 48 months thereafter. At 6 months there was significant mineral loss in all Gruen zones in the three prostheses. By 48 months there were differences in mineral loss between the three prostheses. In the zones adjacent to the porous surface, predominantly zones 1 and 7, the S-ROM exhibited 60% less mineral loss than the Multilock in zone 1, and there was no significant difference in zone 7. Compared to the Multilock-HA, the S-ROM lost 35% less mineral in zone 1, but the Multilock lost 70% less mineral than the S-ROM in zone 7. The Multilock-HA lost 37% and 75% less mineral than the Multilock in zones 1 and 7, respectively, i.e., hydroxyapatite coating tended to preserve bone stalk. Using the Gruen zone area measurements provided by the software, the S-ROM had significantly greater bone resorption in zone 7 at 24 months than either of the Multilocks, which did not differ from each other. In conclusion, DXA has shown differences in periprosthetic adaptive bone remodelling between implants of different design and composition as a function of time.

The osseous response to corundum blasted implant surfaces in a canine hip model

The purpose of this study was to examine the radiographic and histologic response to corundum blasted implant surfaces of varying roughness in a canine total hip arthroplasty model. Three types of tapered femoral implants were made from titanium alloy and were identical in every respect except surface finish. The entire surface of the femoral implant had a 2.9-, 4.2-, or 6.7-micron average surface roughness (Ra) from blasting with 60-, 24-, or 16-grit corundum particles, respectively. Twenty-two stems in 11 dogs were evaluated at 6 months. Twenty-one of the stems showed osseointegration, whereas in one stem a fibrous interface developed. Abundant new periimplant bone formation occurred, particularly within the intramedullary canal where trabeculae spanned implant to endosteal cortex gaps as large as 5 mm. Bone apposition with the 60-, 24-, and 16-grit stems averaged 31.7%, 32%, and 27.9%, respectively; the differences were not statistically significant. However, the pattern of new bone formation was different in that the average length of each region of bone apposition for the 60- and 24-grit surfaces was 50% greater than that for the coarser 16-grit surface. The observations of this study indicate that because of their highly osteoconductive nature, corundum blasted surfaces represent an important and valuable technology for the design of noncemented implants.

Enhancement of bone growth into porous intramedullary implants using non-invasive low intensity ultrasound

An in vivo study was designed to determine if non-invasive low intensity ultrasound could enhance bone growth into porous intramedullary implants. Fully porous intramedullary rods were implanted bilaterally into the ulnae of six dogs. In each dog, one ulna served as a control and the other was treated with 20 min of daily ultrasound stimulation for 6 consecutive weeks. Analysis of serial transverse sections indicated an average of 119% more bone growth into the ultrasound-treated implants compared with the contralateral controls (P < 0.001). In each of the 6 dogs, there was a significantly greater amount of bone ingrowth on the ultrasound-stimulated side. These data indicate a clear potential for externally applied ultrasound therapy to augment biological fixation.

Acid-etched microtexture for enhancement of bone growth into porous-coated implants

We designed an in vivo study to determine if the superimposition of a microtexture on the surface of sintered titanium beads affected the extent of bone ingrowth. Cylindrical titanium intramedullary implants were coated with titanium beads to form a porous finish using commercial sintering techniques. A control group of implants was left in the as-sintered condition. The test group was etched in a boiling acidic solution to create an irregular surface over the entire porous coating. Six experimental dogs underwent simultaneous bilateral femoral intramedullary implantation of a control implant and an acid etched implant. At 12 weeks, the implants were harvested in situ and the femora processed for undecalcified, histological examination. Eight transverse serial sections for each implant were analysed by backscattered electron microscopy and the extent of bone ingrowth was quantified by computer-aided image analysis. The extent of bone ingrowth into the control implants was 15.8% while the extent of bone ingrowth into the etched implants was 25.3%, a difference of 60% that was statistically significant.

These results are consistent with other research that documents the positive effect of microtextured surfaces on bone formation at an implant surface. The acid etching process developed for this study represents a simple method for enhancing the potential of commonly available porous coatings for biological fixation.

Kinematics of the MATCO hip simulator and issues related to wear testing of metal-metal implants

Metal-metal hip implants have been used clinically in Europe to reduce the risk of wear particle induced osteolysis. Joint simulator devices could provide useful information for design improvement of the modern generation of metal-metal hip implants. Early wear results for metal-metal hip specimens were obtained using a MATCO hip simulator. A detailed kinematic analysis was developed for the MATCO simulator and applied to two of the wear experiments to predict the starting surface motion, contact zone and lubricant film thickness. It was shown that points on cup surfaces were not subjected to a reciprocating interaction with the head during wear at the beginning of testing but as wear proceeded, it was suggested that, in some cases, reciprocating interaction did occur on the cup surface. Comparison between simulator and in vivo kinematics suggested a more realistic representation for cup than for head wear. In the simulator, the Hertzian contact zone moved in a circular path over the cup surface and changed in size in correspondence with the applied load. Elastohydrodynamic lubrication was considered to be possible in the simulator, with estimated fluid film thickness as great as 0.1 micron. However, such thick films were not likely to have occurred at the start of the two wear tests which were examined in detail, although some mixed film lubrication might have accounted for the relatively low wear of one of the specimens. The inclusion of kinematic details, contact mechanics and elastohydrodynamic lubrication analysis in simulator testing protocols and in design of metal metal hip implants was recommended.