[The Arthur Vick Award: kinematics of the metacarpophalangeal joint after surface replacement arthroplasty]. / Kinematik des Metakarpophalangeal-Gelenks nach Implantation der Oberflächenersatzprothese.

AIM: Prosthetic replacement in the hand must address such unique challenges as preservation of the collateral ligaments, tendon balancing,and Stability. Surface replacement arthroplasty can be an alternative to other current implants. The purpose of this study was to evaluate the metacarpophalangeal joint kinematics after surface replacement arthroplasty. METHOD: The kinematics of pyrolytic carbon as a surface replacement implant for the metacarpophalangeal joint (MCP) was compared with the intact MCP joint in eight fresh cadaver long fingers by means of an electromagnetic tracking system (Polhemus, Colchester, VT). The eight human cadaver MCP joints were tested before implantation, after implantation, after collateral ligaments resection, and after collateral ligaments reconstruction. RESULTS: The kinematics of the MCP joint is reproduced by the joint surface replacement arthroplasty when normal ligament tension was present. The maximum angular displacement of the pyrocarbon implant was 378 for lateral deviation and 338 for rotation during the passive flexion and extension motion. The instantaneus center of rotation (ICR) after implant insertion was nearly identical to the center of rotation of the normal joint. The results also indicated that the collateral ligaments provide the primary stability of the MCP joint. No significant differences in lateral and rotational stability after surface replacement arthroplasty were noted. While collateral ligaments resection significantly affected the stability of the MCP joint. CONCLUSION: The ICR of the pyrocarbon implant most closely matched that of the intact MCP joint. The pyrocarbon implant provides suitable stability to radio-ulnar deviation and rotational stresses as a resurfacing implant and it simulates the kinematics of the intact MCP joint. By using new materials and taking the anatomical and biomechanical requirements into consideration, the endoprosthesis of the finger joints has created an option to achieve good long-term results. The inadequate results of earlier and current prostheses are a consequence of their mechanical construction and their materials. The success of the new implants could be proven by preferably long-term, controlled studies.

Long-term follow-up of pyrolytic carbon metacarpophalangeal implants.

BACKGROUND: The metacarpophalangeal joint is the most commonly involved joint when rheumatoid arthritis affects the hand. Many prosthetic implants have been designed for the replacement of this joint. Although studies of these implants have shown relief of pain, they have generally demonstrated a poor range of motion, progression of ulnar drift, and bone loss, as well as failure, fracture, and dislocation of the implant. METHODS: From December 1979 to February 1987, 151 pyrolytic carbon metacarpophalangeal implants were inserted in fifty-three patients. The implants had an articulating, unconstrained design with a hemispherical head and grooved, offset stems. Forty-four patients had rheumatoid arthritis; five, posttraumatic arthritis; three, osteoarthritis; and one, systemic lupus erythematosus. Three patients (eleven implants) were lost to long-term follow-up, and twenty patients (fifty-one functioning implants) died after the implant had been in situ for an average of 7.2 years. Eighteen implants (12 percent) in eleven patients were revised. Fourteen of the eighteen implants were replaced with a silicone-elastomer or another type of implant, and the remaining four were removed and a pyrolytic carbon implant was reinserted with the addition of bone cement or bone graft, or both. Twenty-six patients (seventy-one implants) were available for long-term review at an average of 11.7 years (range, 10.1 to 16.0 years) after implantation. RESULTS: The implants improved the arc of motion of the fingers by an average of 13 degrees and elevated the arc by an average of 16 degrees. As a result, fingers were in a more functional, extended position. A complete set of preoperative, postoperative, and follow-up radiographs was available for fifty-three of the seventy-one implants that were followed long term. There was a high prevalence of joint stability: fifty (94 percent) of the fifty-three implants were in a reduced position postoperatively, and forty-one (82 percent) of those fifty implants were still in the postoperative reduced position at the time of long-term follow-up. Ulnar deviation averaged 20 degrees preoperatively and 19 degrees at the time of follow-up, with only the long finger having an increase in deviation. No adverse remodeling or resorption of bone was seen. Fifty (94 percent) of the fifty-three implants had evidence of osseointegration, with sclerosis around the end and shaft of the prosthetic stems. Radiolucent changes were seen adjacent to twelve implants. There was minimum-to-moderate subsidence (four millimeters or less) of thirty-four implants; most of the subsidence occurred immediately postoperatively. Survivorship analysis demonstrated an average annual failure rate of 2.1 percent and a sixteen-year survival rate of 70.3 percent. The five and ten-year survival rates were 82.3 percent (95 percent confidence interval, 74.6 to 88.2 percent) and 81.4 percent (95 percent confidence interval, 73.0 to 87.8 percent), respectively. None of the revised implants had any visible changes of wear or deformity of the surfaces or stems. Four instances of chronic inflammatory tissue and three instances of proliferative synovitis were noted histologically. Focal pigment deposits were seen in three fingers, one of which had removal of the implant two months after a fracture. No evidence of intracellular particles or particulate synovitis was found. CONCLUSIONS: The results of this study demonstrate that pyrolytic carbon is a biologically and biomechanically compatible, wear-resistant, and durable material for arthroplasty of the metacarpophalangeal joint.

Bone remodeling due to continuously applied loads.

It has long been known that the stress history of bone tissue influences its structure; however, the nature of this relationship remains largely uncharacterized. The objective of this work was to induce a quantifiable change in the stress history of in vivo bone tissue and examine subsequent changes in structural and material properties that might occur. Continuous compressive loads were applied to the diaphysis of adult mongrel dogs for 2 months. The loads, ranging from 12-130 N, were superposed on the normal activity of the animals by implanting spring loading devices on the diaphysis of the femur. After the animals were sacrificed, mid-diaphysial specimens were subjected to compression testing to determine a structural bulk stiffness. The cross-sectional areas of original bone tissue and new bone deposition were then determined. The ash weights of selected specimens were also determined. The results indicate that a positive correlation between the increase in cross-sectional area and the superposed stress does exist. The new bone apposition was found almost entirely on the periosteal surface. Very little evidence of internal remodeling or endosteal movement was observed. The new tissue was found to have a lower ash weight and appeared to have a disorganized microstructure. Mechanical testing also suggests that the newly deposited tissue is far less stiff than the mature original bone.

The retention mechanics of LTI carbon, carbon-coated aluminum oxide, and uncoated aluminum oxide dental implants.

The in vitro mechanical behavior of identical geometry LTI carbon, carbon-coated aluminum oxide, and uncoated aluminum oxide blade-type dental implants has been evaluated using rosette type strain gauges and a LVDT system. The implants served as a distal abutment for a three-unit fixed prosthesis and functioned for a period of 2 years in female baboons. The comparison of the LTI carbon implants to the carbon-coated aluminum oxide implants allowed for a study of the effect of implant elastic modulus on the mechanical behavior, while the comparison of the carbon-coated and uncoated aluminum oxide implants allowed for a study of the effect of chemical composition at the tissue-implant interface. The results of the mechanical testing indicate that the implant displacement response of the LTI carbon implants was greater than that of the carbon-coated and uncoated aluminum oxide implants. Little difference in displacement response was observed for the carbon-coated and uncoated aluminum oxide implants. No clear trend in the strain response of the buccal mandibular bone was observed for the three implant systems. The greatest differences in strain response, however, was observed between the uncoated aluminum oxide implants and the LTI carbon and carbon-coated aluminum oxide implants.

Quantitative histologic evaluation of LTI carbon, carbon-coated aluminum oxide and uncoated aluminum oxide dental implants.

The response of mandibular bone to identical geometry LTI carbon, carbon-coated aluminum oxide, and uncoated aluminum oxide blade-type dental implants in baboons for 2 years was evaluated using histologic, microradiographic, and scanning electron microscopic methods. In addition, a quantitative histologic analysis was performed identifying the type, amount, and distribution of tissue surrounding the dental implant systems. This is the final phase of a study investigating the effect of implant elastic modulus and implant surface chemical composition on the performance of dental implants. Previous studies have utilized clinical and radiographic evaluations, postretrieval mechanical testing, and finite element stress analysis to evaluate the dental implant performance. The results of the histologic study revealed a direct implant-bone interface with no intervening soft tissue in 16 of the 21 implants (76%). A fibrous tissue interface was observed in 5 of 21 implants (24%). Quantitative histologic results for the implants with a direct implant-bone interface showed statistically larger crestal cortical plates (p less than 0.05) and greater area fraction crestal cancellous bone (p less than 0.05) in the LTI carbon implant compared to the carbon-coated and uncoated aluminum oxide implants. The carbon-coated and uncoated aluminum oxide implants demonstrated statistically greater area fraction cancellous bone at the inferior region of the implant (p less than 0.05) and thinned and reduced crestal cortical plates when compared to the LTI carbon implants. The results indicate that significant stress shielding of the crestal bone had occurred with the rigid carbon-coated and uncoated aluminum oxide implants when compared to the LTI carbon implants which had a material elastic modulus similar to cortical bone. Based upon the histologic results, it appears that the LTI carbon implants with the direct implant-bone interface exhibited a greater potential for long-term successful performance compared to the aluminum oxide substrate implants.

Pyrolite carbon implants in the metacarpophalangeal joint of baboons.

A nonconstrained uncémented Pyrolite carbon prosthesis was evaluated for replacement of the metacarpophalangeal joint. Six prostheses were inserted into the long finger metacarpophalangeal joint of four baboons. Nine months after insertion, the prostheses and surrounding tissues were removed enbioc and evaluated radiographrca/ly, utilizing histologic and microradiographic analyses. The four Pyrolite carbon implants inserted without cement were well tolerated. Histological evidence of direct appositional bone fixation along the medullary stem was observed in one specimen, and a combination of bone fixation with an interposing fibrous tissue membrane was observed in another. There was no evidence of bone resorption around the implant stems, and functional fixation was obtained with all of the uncemented Pyrolite carbon implants. No foreign body reaction was observed in the soft tissues, and no evidence of intracellular particles was present. Two cemented implants (one Pyrolite carbon and one polyethylene and metal) were also evaluated; both showed evidence of bone resorption and/or gross implant loosening.This study has demonstrated the potential for biological fixation with Pyrolite carbon-stem med implants for prosthetic replacement of the metacarpophalangeal joint, which thus offers significant improvement as a material for joint reconstruction.

Parameters affecting the stress distribution around LTI carbon and aluminum oxide dental implants.

Three-dimensional finite element stress analysis was used to study the effects that implant neck geometry and the tissue properties at the implant-bone interface have on the stress distribution around free-standing blade- and post-type LTI carbon and aluminum oxide dental implants. Implants having neck flares of 13 degrees and 26 degrees were studied. In addition, to simulate fibrous encapsulation of the implant as opposed to a direct bone apposition retention mechanism, a soft tissue interposing layer between implant and bone was also modeled. The results of the study indicate that a reduction in neck flare from 26 degrees to 13 degrees was a positive design change for blade- and post-type LTI carbon implants and blade-type aluminum oxide implant but not for post-type aluminum oxide implant. The results of the study indicate the presence of fibrous tissue surrounding the implants may be indicative of a failing system and may be the result of either hypophysiological stress (aluminum oxide implants) or hyperphysiological stress (LTI carbon implants).

A clinical comparison of LTI carbon, alumina, and carbon-coated alumina blade-type implants in baboons.

The clinical performance of LTI carbon, carbon-coated aluminum oxide, and uncoated aluminum oxide blade-type dental implants was studied in baboons. The objective of the study was to determine the effect that implant material elastic modulus and surface composition have on implant performance. Clinical parameters of mobility, sulcus depth, soft tissue characteristics, and radiographic appearance were used in the evaluation. The implants were placed in healed extraction sites in adult female baboons and were used as a distal abutment for a three-unit fixed gold prosthesis. The restorations were allowed to assume normal occlusal function and were left in situ for a period of two years. The radiographic and sulcus depth measurements appeared inferior for the LTI carbon implants and best for the carbon-coated aluminum oxide implants. No differences in mobility or soft tissue characteristics were noted for the three implant systems. Two implants both in the same animal--one LTI carbon and one uncoated aluminum oxide--were definite clinical failures. The results of the study indicate that an elastic modulus mismatch between implant and bone is not an a priori cause of implant failure and that the implant surface composition had little apparent effect on the clinical and radiographic performance of these implant materials.

A model for the implant-bone interface characteristics of porous dental implants.

The use of computer-aided design techniques, such as finite element stress analysis, to evaluate implant designs requires that the mechanics at the interface between the implant and the bone be characterized. This problem is accentuated when implants designed to achieve stabilization by tissue growth into porous coatings are used. A model has been developed to represent the mechanical properties of the porous material-bone interface. The model, which accounts for the mechanics of bone tissue grown into a porous implant surface, was also developed so that it could be incorporated into a three-dimensional finite-element analysis. The analytical results from a three-dimensional finite-element model of a cylindrical porous rooted dental implant system incorporating the interface model were compared to the results of mechanical tests performed on similar implants which had resided in canines for two yr. The results from the finite element analysis using the interface model were found to be in better agreement with the experimental results than when a direct bone-to-porous-material interface was assumed.

The influence of implant geometry on the stress distribution around dental implants.

A three-dimensional finite element stress analysis has been used to investigate the influence that variations in the infrastructural geometry of a blade-type dental implant have on the stress distribution around LTI carbon and aluminum oxide implants. The finite element model was constructed based upon an analysis of serial sections of a retrieved implant specimen. In addition to the implant, the finite element model contained a three-unit fixed bridge connected to a natural molar with periodontal membrane. The removal of the bridge allowed for the study of freestanding implants and molar. Variations of the implant blade geometry were found to produce significant changes in the stress distributions around bridged and freestanding aluminum oxide implants. Very little effect, however, was observed around the LTI carbon implants. A comparison of the stresses around the freestanding molar and the stresses around the bridged and freestanding implants was made to determine the implant design that came closest to reproducing the stress state around the modeled molar. The LTI carbon system that best achieved this stress state was found to be a full-blade implant used in conjunction with a tooth as an abutment in a fixed bridge. The aluminum oxide system that best achieved this stress state was found to be of the post or short-blade design used as a freestanding implant.

Retention characteristics of porous rooted Co-Cr-Mo alloy dental implants.

The interface characteristics of porous rooted cobalt-chromium-molybdenum alloy (Co-Cr-Mo) dental implants which had been in free standing function in canine mandibles for a period of two years were investigated. The displacement of the implants and of points on the adjacent mandibular cortex were determined by mechanical testing. Bone ingrowth was quantified, and the structure of the bone-implant interface and mandibular cortex were characterized using histologic and microradiographic analyses. Displacement characteristics were correlated with determinations of the tissue structure adjacent to and growth within the implant to provide information about the biological attachment. A correlation was found between the thickness of the buccal and lingual cortical plates and implant displacements; implants having the greatest displacement response were in mandibles with the thinnest cortical plates. A relationship could not be established between the implant displacement response and the quantitative tissue structure data. Differences observed in the displacement response of the implant by mechanical testing were not observed by clinical measurements of mobility. It was concluded that implant retention mechanical behavior results from both interfacial displacement and deflection of the adjacent mandibular structures.

The effects of intramedullary implants on bone strains and remodeling in the femur.

Bone remodeling and strain distribution in the femur was studied after implantation of LTI pyrolytic carbon, bioglass coated Co-Cr-Mo alloy, and carbon coated porous Co-Cr-Mo alloy intramedullary plugs. Mechanical testing and finite element analysis has shown that the intramedullary stems significantly alter the strain pattern in the femur. However, no statistically significant differences were observed among the implant groups. The analytical and experimental results were found to be in good agreement. Radiographically, differences were observed in the bone remodeling around the carbon implant compared to either of the Co-Cr-Mo based implants. Both internal and external bone remodeling takes place when an intramedullary implant is present.

The influence of the bone-implant interface stiffness on stress profiles surrounding Al2O3 and carbon dental implants.

Dental implants have been used and studied for the replacement of missing teeth for many years. Finite element stress analysis (FESA) has previously been used in their evaluation to study the effect of various design parameters on induced stresses. A two-dimensional FESA was used to evaluate the effect that the implant-bone interface elastic modulus has on the stress distribution around LTI carbon and aluminum oxide dental implants. The results of this investigation indicate that a soft tissue interface between implants and bone negates the effect of implant elastic modulus and results in stress profiles that were almost identical for the LTI carbon and aluminum oxide implants.

A three-dimensional finite element analysis of a porous rooted Co-Cr-Mo alloy dental implant.

A three-dimensional finite element model was used to investigate the biomechanical response of a porous rooted Co-Cr-Mo alloy implant. The load displacement responses calculated from the finite element model were verified with experimentally-determined values for three-cylindrical, porous rooted Co-Cr-Mo alloy dental implants retrieved from canines after two years in function. Models were developed based on histological analyses of tissue surrounding the implants. The results of the study indicate that the assumption of a direct bone-to-implant interface (i.e., an ankylosed implant) may not be a good representation for a porous rooted implant retained by bone ingrowth.

Retrieval and analysis of intramedullary rods.

UNLABELLED: We examined ten intramedullary rods of similar design after routine retrieval from patients. Of these ten rods, four were found to exhibit cracking around their proximal third. This behavior could not be attributed to the surgical techniques employed or to the length of time in vivo of these rods. Rather, the cracking was a function of both the metal alloy used and the method of manufacture, which occasionally allowed a weld zone to be located at the point of maximum stress with the result that cracking occurred. A change in alloy composition to a low-carbon form of 316 stainless steel probably would reduce the risk of cracking. CLINICAL RELEVANCE: In the treatment of orthopaedic disorders, it is important for the operating physician to appreciate the problems that may be encountered in using implants. One of the most important of these problems is the possibility of implant failure. The present report illustrates how a combination of both metallurgical and fabrication factors may cause such an event to occur.

The influence of implant elastic modulus on the stress distribution around LTI carbon and aluminum oxide dental implants.

A three-dimensional finite element analysis (FEA) has been used to determine the effect of implant elastic modulus on stresses in tissues around LTI carbon and aluminum oxide dental implants. The finite element model was constructed to represent a baboon mandible containing a blade type dental implant. A three unit fixed bridge was modeled connecting the dental implant to a natural molar. The results of the study indicate that stress levels of approximately a factor of 3 lower in the crestal region can be expected for aluminum oxide implants when compared to the LTI carbon implants. It was also observed that the use of LTI carbon and aluminum oxide dental implants as an abutment in a fixed bridge results in a reduction of stresses in tissues around the natural tooth when compared to normal physiological stress levels.

An evaluation of ion-textured aluminum oxide dental implants.

A study was undertaken to evaluate the ion-beam texturing of aluminum oxide as a means of providing a surface which will produce a biological prosthetic attachment. A wafflelike pattern of surface contours 150 x 75 x 35 microm deep was produced on cylindrical dental implants. The textured surfaces were compared to the as received surfaces in adult mongrel dogs. Implants were inserted into surgically modified healed extraction sites and were left in place for six months. Post-sacrifice mechanical testing was used to quantify the displacement response of the implants. The clinical, radiographic and mechanical testing evaluations did not reveal any statistically significant differences in the performance of the dental implants. However, it was observed that anatomical site and mandibular geometry with respect to implant size play a significant role in affecting implant retention.

Interface mechanics of porous titanium implants.

The interfacial shear properties of bone tissue growth into porous coated Ti-6-A1-4V femoral implants have been examined as a function of the pore size of the porous surface. Three particle size range powders (297 microns, 420-500 microns, 595-707 microns) were used to fabricate cylindrical implants which were inserted into the femoral medullary canal of dogs for 6 months. Push-out tests on the removed femurs are reported and reveal: (i) that those implants residing in cortical bone exhibited significantly higher shear properties than the equivalent implants in cancellous bone and (ii) that the interfacial shear strength and stiffness decreased with increasing pore diameter within the range 175-325 microns. The extent of bone ingrowth into the surface of the implants was measured using quantitative optical microscopic techniques. This indicated that the percentage of bone which had grown into the surface was inversely proportional to the square root of the pore size and that further the shear properties of the interface were proportional to the extent of bone ingrowth.

Demonstration of probes in human periodontal pockets.

Twenty-two anterior and bicuspid teeth that previously had been designated for extraction, were removed from 11 male patients. Two teeth were extracted from each patient and each tooth had a periodontal pocket of at least 4 mm and a Gingival Index of 2 or 3. One tooth in each patient was subjected to scaling, root planing, and plaque control in an effort to reduce gingival inflammation. By means of an orthodontic tube and composite resin bonding material a periodontal probe under 15 to 20 gm of pressure was fixed in the periodontal pocket of each tooth. The specimens were removed en bloc and histologic specimens prepared. The conclusions were that when the gingiva is inflamed, the tip of the periodontal probe tends to extend to the apical base of the junctional epithelium or slightly beyond but there is great variation in this position. Further, it was concluded that the position of the probe tip during probing is not affected by the depth of the sulcus or periodontal pocket.