Rotational constraint in posterior-stabilized total knee prostheses.

Rotational stresses from box-post impingement have been implicated in the loosening of posterior-stabilized total knee prostheses. A bench model was constructed to assess the forces generated by tibiofemoral rotation. Rotational torque under load was measured in two different posteriorstabilized total knee prostheses using an axial-torsion load cell at 0 degrees, 20 degrees, and 40 degrees flexion over 20 degrees internal and external rotation. The Sigma posterior-stabilized prosthesis generated little torque through 5 degrees internal and external rotation. An increase in torque then occurred because of box-post impingement, generating peak torques of 17 to 18 N-m at 12 degrees to 14 degrees rotation. The bench model produced the same deformation of the polyethylene post as seen on retrieved specimens. The Scorpio posterior-stabilized prosthesis had a relatively continuous rise in generated torque from tibiofemoral conformity. Box-post impingement did not occur resulting in 32% lower torque between 12 degrees and 14 degrees rotation. Peak rotational torques of 15 to 16 N-m were reached at 19 degrees to 20 degrees rotation. Tibiofemoral conformity is the primary source of rotational constraint. Box-post impingement can be a source of additional rotational constraint. Depending on specific design features, small changes in relative tibiofemoral component rotation can more than double the generated torque. Axial rotation of the knee in vivo can generate substantial torque. Relative tibiofemoral rotational position is an important factor influencing component function and fixation.

Wear and corrosion of modular interfaces in total hip replacements.

Modular components allow for the customization of hip replacements to the individual patient. Modular head-neck components allow for mixed material systems to minimize polyethylene wear as well as provide the ability to vary neck length and head size independent of the stem. Modular interfaces, however, result in an increased susceptibility to interface corrosion and wear debris generation. One hundred eight uncemented femoral stems with modular heads retrieved for reasons other than loosening with modular heads were examined for interface corrosion. In addition, in an effort to quantify the amount of wear debris generated at modular interfaces due to cyclic loading, mechanical testing and electrozone particle analysis was used to study various surface, material, and design combinations. Detectable degrees of corrosion were observed in ten of 29 (34.5%) mixed alloy systems and seven of 79 (9%) single alloy components at an average of 25 months in situ. There was no correlation between presence or extent of corrosion or surface damage with time in situ, initial diagnosis, reason for removal, age, or weight. Stems with corrosion were less likely to have bone ingrowth histologically. The results of mechanical testing showed a significant number of wear particles were generated by all head-neck combinations. The wear debris was almost totally in the size range less than 5 microns. As many as 2.5 million particles were generated the first million cycles loading, with as many as eight million particles generated at ten million cycles. The results indicate that surface preparation and material affect particle generation. Head-neck tolerance mismatch appears to be significantly variable in the number of particles generated.

Torsional resistance and wear of a modular sleeve-stem hip system.

The torsional resistance and wear debris generation of the modular sleeve and stem S-ROM total hip system was evaluated. The results indicate that slippage of the sleeve-stem interface may occur under physiological loading conditions. Slippage is more likely to occur if the junction is contaminated by blood or tissue, or if the stem is disengaged and reimplanted into the sleeve. Significant wear debris was generated during cyclic fatigue loading. The wear debris was of the size (less than 10 mum) readily ingested by macrophages. Particles of these dimensions have been associated with osteolysis, implant loosening and pain. Based upon the findings of this study the implantation of this type system must be carefully considered.

Accuracy of clinical isometry and preload testing during anterior cruciate ligament reconstruction.

To determine the effect of suture elongation on the accuracy of tension isometers, an anatomic specimen was used to measure peak loads on suture materials placed in extreme nonisometric positions. Accuracy and reproducibility were determined for three tension isometers, and the influence of suture elongation was determined for one of them. In one device, reproducible readings ranged from -12.3% to + 23.6% of mean static loads recorded by hydraulic testing equipment; average error approached 39.1%. Values tended to be overestimated if readings ranged as high as 32.5% over specific excursion ranges. Selected suture materials were subjected to these loads, using hydraulic testing equipment. Stainless steel exhibited extremely low levels of elongation but only at large diameters. Ethibond had similar low levels of elongation but only at large diameters. Monofilament nylon exhibited significant elongation that could cause erroneous readings and nonisometric placement. The measurement tolerances of a selected tension isometer should be considered by the surgeon performing anterior cruciate ligament (ACL) substitution. Several readings, both loading and unloading, can be used to determine preload and isometry. Material properties of the selected suture and the accuracy of the tension isometer should be considered when interpreting readings from isometric positioners. This brings into question the ability of currently available tension isometers to deliver readings within prescribed tolerances for either isometry of preload testing.

Wear characteristics of the canine acetabulum against different femoral prostheses.

We studied cartilage degeneration in 45 canine acetabula after implantation of prostheses with articulating surfaces of low-temperature isotropic (LTI) pyrolytic carbon, cobalt-chromium-molybdenum alloy and titanium alloy for periods ranging from two weeks to 18 months. Gross specimens and histological sections were compared with the nonoperated (control) acetabulum of the same animal. Cartilage articulating with LTI pyrolytic carbon exhibited significantly lower levels of gross wear, fibrillation, eburnation, glycosaminoglycan loss, and subchondral bone change than with metallic surfaces. Survivorship analysis showed a 92% probability of survival for cartilage articulating with LTI pyrolytic carbon at 18 months, as compared to only a 20% probability of survival for cartilage articulating with either of the metallic alloys.

Comparative mechanical analysis of a looped-suture tendon repair.

The in vitro breaking force of a braided nylon looped-suture tendon juncture designed to decrease tying time was compared with the breaking force of the modified Kessler and Bunnell techniques. Repaired with either braided nylon or tetrafluoroethylene, porcine digiti quarti propius tendons were tested to single cycle failure on a MTS hydraulic testing machine. The results showed that the looped-suture technique had a mean breaking force that was statistically indistinguishable from that of the Bunnell technique regardless of suture material. However, the breaking forces for the looped suture and Bunnell techniques were statistically greater for both suture materials when compared with the modified Kessler technique. The resistance to gap formation for the looped suture was found to be intermediate between the Bunnell technique and the modified Kessler technique.

An evaluation of the mechanical failure modalities of a rotating hinge knee prosthesis.

The clinical and metallurgical performance of 12 retrieved Noiles total knee prostheses was investigated. Clinical histories and serial roentgenograms were used in conjunction with the American Society for Testing and Materials (ASTM) standard implant retrieval and analysis techniques to assess device performance. All of the devices were removed because of implant loosening with pain, while two devices also had a late infection. The average time in situ for all devices was four years. Roentgenographically, all of the devices showed signs of progressive radiolucencies. Ten (83%) of the devices migrated proximally within the femur. A marked varus drift was observed in eight (67%) of the retrieved prostheses. Hypertrophy of the femoral cortex was observed in eight (67%) of the cases. Seventy-three percent of the polyethylene tibial stems exhibited extensive wear on the anterior and medial aspects of the articulating surface. Extensive wear was also seen on the anterior and extreme posterior aspects in 92% of the polyethylene tibial plateau bearings. This wear was a consequence of the migration of the femoral component leading to buttressing of the plastic components against bone or bone cement. This study indicates serious design flaws in the Noiles knee prosthesis that unless corrected would question the use of the device in either primary or revision knee surgery.

A chronically dislocating prosthetic patella. A case report.

A 63-year-old white woman was evaluated for a chronically dislocating right patella 6 months following total knee arthroplasty. At the time of our initial evaluation, her knee would not extend beyond 35 degrees and her patella was dislocated and irreducible. At arthrotomy, when the patella was everted, the patellar component was oriented with the facet ridge rotated 90 degrees to the trochlear groove of the femoral component. The patellar template guide indicated that the fixation pegs had been drilled properly, and thus, the patellar component had been assembled improperly during manufacturing. After dome-type patellar prosthesis replacement and proximal and distal extensor realignments were performed, 2 months post-revision the patient had a range of motion from 5 degrees to 95 degrees and ambulated with no patellar instability or pain.

Tissue response to porous-coated implants lacking initial bone apposition.

Although initial bone apposition of a porous-surfaced implant is desirable, it is not always achieved surgically. A model to study the effect of a gap on the quantity and quality of bone growth in both the cancellous and cortical regions has been developed. Implants were surgically placed in the intramedullary canals of adult dogs producing uniform gap spaces 0.0-2.0 mm wide. Histologic and microradiographic evaluations were conducted after 3, 6, and 12 weeks in situ. The results demonstrate that the initial apposition of a porous implant to the surrounding bone surface is not necessary for fixation by bone ingrowth. New bone will grow up to and within the porous structure of an implant even when there is a gap as large as 2.0 mm. However, the rate and degree of maturity and mineralization is enhanced when the gap width is 0.5 mm or less. The amount of bone activity in the cortical region was greater than in the cancellous region at 3 and 6 weeks after operation. After 12 weeks in situ bone growth in gap spaces and into the porous coating was approximately equal.

Fatigue failure of the femoral component of a unicompartmental knee.

A unicompartmental knee implant was retrieved after 11 years in situ. The cast stainless steel femoral component had fractured approximately 1 cm from the anterior tip. The implant and clinical tolerance were analyzed to ascertain the factors associated with the mechanical failure. Increased tensile stresses owing to poor surgical placement and wear on the weight-bearing surface of the component, together with the quality of implant material, influenced the mechanical failure. Examination of the fracture surface revealed fatigue to be the mode of failure. Microscopic analysis of sections through the specimens showed a high inclusion content and inhomogeneous grain size, typical of the cast stainless steel from which the femoral component was fabricated.

Biomechanical evaluation of superficial transfer of the biceps femoris tendon.

The biomechanics of the biceps femoris tendon transfer were determined from four fresh, frozen extremities. All transfers involved the proximal and anterior advancement of a selected portion of the tendon to a position superficial to the lateral collateral ligament. The rotational torque and flexion force were measured before and after the transfer with the knee positioned at 10 degrees, 20 degrees, 30 degrees, and 45 degrees of flexion, in neutral and 5 degrees internal rotation. Although more specimens need to be tested for statistical confirmation, the transfer of the superficial portion of the biceps tendon resulted in average decreases of 2% and 15% in the rotational and flexion capacity of the biceps, respectively. Biomechanically, the functional lever arm of the transferred tendon about the longitudinal (internal-external rotation) axis was essentially unchanged, while the functional lever arm about the sagittal (flexion-extension) axis was decreased slightly. The total transfer of the biceps tendon resulted in an increase of 28% in the rotational capacity, while the flexion capability was decreased an average of 75%. In addition, the total transfer resulted in the biceps becoming an extensor at 10 degrees and 20 degrees of flexion, which could actually worsen the instability. Thus, following the total transfer, the functional level arm about the sagittal axis was decreased, while the functional lever arm about the longitudinal axis was increased. In summary, neither the partial nor the total transfer of the biceps femoris tendon superficial and anterior to the fibular collateral ligament appeared to be a biomechanically effective ancillary for the treatment of anterolateral rotatory instability.

Clinical performance of endoprosthetic and total hip replacement systems.

A retrospective study of endoprosthetic replacements and total hip prostheses was undertaken to determine factors that have the greatest effect on the success or failure of femoral hip components. A total of 227 endoprostheses were inserted within the years 1970-1985. Of these, 67 components (29.5 percent) required removal for various reasons. For an endoprosthesis, the most significant factor in determining the chances of success appeared to be the initial insertion diagnosis. Patients who received an endoprosthesis for an ailment which affected only the femoral side of the joint (such as traumatic fracture) had a much lower rate of failure than those patients with disease etiologies that could affect the acetabulum (such as osteoarthritis). The opposite was found for total hip prostheses. Of the 641 total hip devices inserted, 148 (23.0 percent) required removal. The highest rate of failure among these total hip components was for those devices inserted for trauma and the lowest rate of failure was for those inserted for osteoarthritis. Age at the time of insertion also proved to be of importance when estimating a hip component's chance of survival. For both endoprosthetic replacements and total hip arthroplasties, patients younger than 50 years of age at insertion experienced a failure rate almost twice that of those patients more than 50 years of age at insertion.

A histologic and mechanical evaluation of carbon-coated polyester suture.

Ultra low temperature isotropic (ULTI) carbon-coated polyester suture material was evaluated histologically and mechanically in dogs. These results were compared to those obtained for uncoated polyester and polybutylate coated polyester. The suture materials were used in the repair of the surgically incised medial collateral ligament and subcutaneous tissues to evaluate the potential of the carbon-coated system for ligamentous repairs. Following surgery, the dogs were sacrificed at periods of 1-48 weeks postoperatively for evaluation of tissue biocompatibility and mechanical strength of the materials. The polybutylate-coated polyester suture broke at lower force levels than did comparable sizes of uncoated or carbon-coated polyester. All three types showed a high retention (greater than 98%) of mechanical strength at 48 weeks, often exhibiting an increase in tensile strength due to tissue ingrowth. The histologic response to carbon-coated polyester was equal to or better than the response to either the uncoated polyester or polybutylate-coated polyester. A greater degree of tissue growth into the carbon-coated material was evident at most time periods following an initial acute inflammatory response which was also present in the other materials.

Metallurgical analysis of five failed cast cobalt-chromium-molybdenum alloy hip prostheses.

The clinical and metallurigical characteristics of five cast cobalt-chromium-molybdenum alloy femoral hip prostheses which failed in vivo were evaluated. The devices included: two of the Howmedica Muller-Charnley design, two of the Howmedica Charnley design, and one of the Zimmer Aufranc-Turner design. Fractographic analyses demonstrated that the five devices had failed by fatigue which originated on the lateral aspect. Failure occurred after an average in vivo time of 80.4 months (approximately 6.7 years). Only two of the devices had Rockwell hardness values that were within the ASTM specifications for the alloy. Upon metallurgical examination, moderate to severe levels of gas porosity, interdendritic shrinkage, and nonmetallic inclusions were found in all of the devices. As expected, extremely large grain sizes also were observed in the devices examined. These results indicate that the metallurgical flaws and defects associated with the cast cobalt-chromium-molybdenum alloys used in these devices may preclude successful longterm performance and warrant manufacturer's attention.

The effect of the radial head and prosthetic radial head replacement on resisting valgus stress at the elbow.

Five fresh nonembalmed elbows were tested for resistance to valgus stress in their anatomic state, after radial head resection, and after insertion of Silastic and polymethylmethacrylate (PMMA) radial head replacements. The resistance to valgus stress was found to be reduced an average of 28% after radial head resection. The PMMA and Silastic implants restored valgus stiffness an average of 86% and 78% respectively, as compared to intact elbow values for corresponding flexion angles. Testing in pronation, supination and neutral forearm rotation demonstrated no difference in valgus stiffness. For each elbow, resistance to valgus stress was greatest at full extension and dropped approximately 30% at all other flexion angles as compared to corresponding full extension value. These data support the concept of the radial head as a stabilizer to valgus stress in the in vitro elbow. Further, this data demonstrated the ineffectiveness of current radial head replacement systems in restoring this biomechanical function and suggest that the use of a stiffer implant material may be beneficial in resisting valgus stress. Additional testing is indicated to determine the performance of a stiffer implant at the clinical and biological levels.

Biomechanics of running shoe performance.

The running shoe provides shock absorption and motion control for the runner. The change in shock-absorption properties has been studied as a function of miles run. Different models of running shoes encompassing various manufacturers' models and price ranges were mechanically tested to simulate the repeated heel strikes of running. The energy absorbed by the shoes was determined from the area under the load deformation curve at various intervals to the equivalent of having run 500 miles. Shoes were also tested at similar intervals after having been worn by volunteers during normal training. A difference of approximately 33 per cent in the initial shock absorption was observed in the different shoe models tested. In general, the mechanically tested shoes retained approximately 75 per cent of their initial shock absorption capability after 50 miles and approximately 67 per cent after 100 to 150 miles. Between 250 and 500 miles, the shoes retained less than 60 per cent of their initial shock-absorption capacity. The shoes tested after having been worn during running by volunteers had similar but less severe degradation characteristics. These shoes tended to retain approximately 70 per cent of the initial shock absorption characteristics were apparent based upon either shoe price or manufacturer. In addition, shoes that exhibited superior initial shock absorption capability often had more rapid degradation characteristics.

Shock absorption characteristics of running shoes.

The change in shock absorption properties of running shoes was evaluated as a function of miles run. Different models of running shoes encompassing a wide range in retail price were obtained and mechanically tested to simulate the repeated heel strikes of running. The energy absorbed by the shoes was determined from the area under the load deformation curve at the equivalent of 0, 5, 10, 25, 50, 75, 100, 125, 150, 200, 250, 300, and 500 miles of running. Shoes were also tested at similar intervals after having been worn by volunteers during normal training. An approximate 33% difference in the initial shock absorption was observed in the different shoe models. In general, the shoes retained approximately 75% of their initial shock absorption capability after 50 miles of simulated running, and approximately 67% after 100 to 150 miles. Between 250 and 500 miles the shoes retained less than 60% of their initial shock absorption capacity. No differences in shock absorption characteristics were apparent based upon either shoe price or the manufacturer model. The results of shoes tested by the volunteer runners also showed a marked reduction in shock absorption with mileage. The loss, however, was not as great as in the machine-simulated running, with approximately 70% of initial shock absorption retained at 500 miles.