Paul D. Postak

The Effects of Shelf Life on Clinical Outcome for Gamma Sterilized Polyethylene Tibial Components

Tibial component shelf life was examined as a contributory factor of the in vivo failure of gamma sterilized prosthetic knee replacements. One hundred eighty-eight Synatomic total knee replacements sterilized by gamma irradiation in air were implanted by one surgeon into 147 patients between May 1985 and December 1994. Of these, 135 knees in 105 patients with a mean followup of 5.8 years (range, 2.1-11.3 years) were included in the study. The mean shelf life of the implants was 3.6 years (range, 0.1-10.7 years). Clinical failure for this study was defined as component retrieval resulting from polyethylene degradation. The knee components were divided into three different groups determined by their shelf storage durations of 0 to 4 years (Group 1, 93 components), between 4 and 8 years (Group 2, 21 components), or greater than 8 to 11 years (Group 3, 21 components). Six prostheses were revised because of polyethylene degradation after a mean implantation time of 2.5 years (range, 1.1-3.8 years). The mean shelf life of these six prostheses was 8.4 years (range, 5.8-9.6 years). Five years after implantation, prostheses that had shelf lives of less than 4 years had a 100% survival rate. Those that had shelf lives of 4 to 8 years before implantation had an 88.6% survival rate, and those prostheses that had shelf lives greater than 8 to 11 years had a 79.2% survival rate.

A comparison of the disassociation strength of modular acetabular components.

Five short-term in vivo disassembly of two-piece acetabular cup designs have been reported. This study evaluates the liner retention strengths of eight contemporary cup systems. Both push-out (663 +/- 65.5 pounds force to 29 +/- 1.4 pounds force) and lever-out (684 +/- 114 inch-pounds to 43 +/- 1.5 inch-pounds) test modes show a wide variation in retention strength. Repeat liner separation testing demonstrates a 26% and 32% respective decrease in locking mechanism integrity. These findings indicate that reseating modular liners at the time of surgery or reassembling a previously separated liner should be avoided.

The biomechanical effects of geometric configuration of bone-tendon-bone autografts in anterior cruciate ligament reconstruction

This study provides biomechanical support for a new technique of autograft anterior cruciate ligament reconstruction featuring circular bone plugs and endosteal interference fit fixation. Six matched pairs of fresh frozen human knees were utilized. Femoral interference fit pull-out strength was determined from material-testing-machine-generated oscillograph recordings at a strain rate of 100%/s. Circular bone plugs, obtained with a circular oscillating saw, provided 19.9% greater interference fit pull-out strength compared with identically fixed trapezoidal bone plugs. Different geometric defects were compared in three- and four-point bending on an Instron machine with frozen patellae and an artificial bone composite. Circular defects have 107% greater strength than matched trapezoidal patellar defects in three-point bending. In a bone composite, circular defects are 53% stronger than triangular and 25% stronger in four-point bending than trapezoidal defects. A new technique of harvesting bone plugs with endosteal interference fit fixation is described and biomechanically supported. To date, this technique has been performed on over 500 cases clinically without evidence of patellar fracture or fixation failure. This study demonstrates the efficacy of this simple and reproducible technique compared with previously reported procedures.

Classification of mobile-bearing knee designs: mobility and constraint.

Restoration of normal knee joint function through surgical reconstruction is dependent upon load-sharing between the implant and the surrounding soft-tissue structures. Mobile-bearing knee designs offer the advantage of maximally conforming geometry while diminishing constraint forces to fixation interfaces through plateau mobility. The degree of mobility afforded by these designs in the anterior-posterior, medial-lateral, and rotational directions defines the required interaction between the soft tissues and the design geometry to maintain a stable articulation. This study characterizes nine contemporary mobile-bearing designs in terms of the force generated during a prescribed displacement. Among the designs evaluated, only the LCS Deep Dish Rotating Platform is available for clinical use in the United States. A dynamic testing system capable of applying biaxial loads (Instron Testing Machine, model 1115; Instron, Canton, Massachusetts) was utilized to assess the intrinsic performance characteristics of nine non-hinged, mobile-bearing knee designs. Anterior, posterior, medial, lateral, and rotational constraints were determined for each total knee design under a compressive load consistent with normal walking gait1,2. A compressive load of four times body weight and 0° of flexion was chosen to represent a position of gait where maximum shear forces act in the posterior and lateral directions as well as in rotation1,2. Anterior and medial shear forces are presented at the same gait position for completeness. A body weight of 163 lb (74 kg), which corresponds to the average for a sixty-year-old, 5-ft, 8-in (172.7-cm) male subject, was used in this evaluation3. ### Anterior-Posterior and Medial-Lateral Shear Testing Three tibial inserts were evaluated in each test direction for each system. Under the prescribed in vivo compressive load (652 lbf), shearing displacements were applied to the system until the implant subluxated (Fig. 1). Anterior, posterior, medial, and lateral subluxation was defined as the dislocation of the tibial component relative …

Biomechanical analysis of pin placement and pin size for external fixation of distal radius fractures

A series of biomechanical analyses were performed to explain the recent reduction in treatment-related complications of external fixation of distal radius fractures using a limited open approach for pin placement and larger 4-mm self-tapping half pins. A comparison of pull-out strength, stress concentration effect, and inherent bending strength of 3− and 4-mm half pins was performed. The effect of proximal pin placement in the radius or in the ulna and the effect of distal pin placement in four, six, or eight metacarpal cortices were determined. These analyses demonstrate that the 4-mm self-tapping half pins result in a significantly higher pull-out strength and only a small decrease in torsional load strength of the bone. They also demonstrate that proximal pin fixation in the radius produces the most stable fixation and that distal pin fixation into six metacarpal cortices produces a strong configuration that does not violate the interosseous muscles of the second intrinsic compartment. The rate of treatment-related complications in the external fixation of distal radius fractures (specifically, pin loosening, bending and breakage, fracture through pin sites, collapse at the fracture site, and intrinsic contracture) are addressed in this study. Such complications can be minimized by using 4-mm pins after central predrilling, with proximal placement in the radius and distal placement through six cortices of the bases of the second and third metacarpals.

The effects of external torque on polyethylene tibial insert damage patterns.

The forces and torques that occur during walking gait, particularly during toe-off, promote articulation in the posteromedial quadrant of tibial inserts. Retrieved components of failed knee arthroplasties show ultrahigh molecular weight polyethylene damage patterns in this region. Component-designed constraint, compromised polymer, and surgical factors account for these observations. The current authors compare the contact stresses that developed on four implant designs during toe-off for optimally aligned and externally torqued components using the finite element method. Under 16 N-m of torque, the four designs studied varied regarding their centers of rotation and magnitude of external rotation, which are related directly to their specific articulating surface geometry. Designs with conforming condylar geometry had greater rotational constraint and therefore, less external rotation. These conforming designs offer the benefits of lower stresses and tend to limit contact near the edge of the plateau. However, because of their increased rotational constraint, torque is transmitted more readily to the implant-bone interface, increasing the potential for implant loosening. The data presented serve as an indicator of the potential for polyethylene tibial component surface damage and define the role that implant geometry plays in resisting external rotation.

The current state of cervical and lumbar spinal disc arthroplasty

The growth of spinal implant and orthobiologic technologies over the last several years has been increasing in tempo1 (Fig. 1) and is fast approaching the size of total hip and knee arthroplasty in annual dollar sales in the United States. During this time, a number of start-up and established medical device manufacturers have focused increasing resources on solutions for spinal problems. The role of the orthopaedic and neurosurgeon in these enterprises as inventor, owner, and user has contributed to this march of progress. This paper describes a small (<1%) but increasingly visible aspect of these advancing technologies, that is, artificial disc replacement. The etiology of mechanical back and neck pain is not well understood and is among the more difficult problems encountered by the spine surgeon. Specific pain generators may include the disc as well as the facet joints at any given level or at multiple levels of the spinal column. Treatment goals, not unlike those of hip and knee reconstruction, are to eliminate pain, maintain or restore stability, correct height loss, and, in the case of disc arthroplasty, offer motion preservation. Fusion is the accepted gold standard of treatment, but it lacks the latter prospect of intervertebral mobility. Successful patient outcomes for both lumbar and cervical fusion have ranged from 60% to 85%, but complications of adjacent spinal disc disease, incomplete pain relief, and morbidity arising from the surgical approach have been reported2-10. Fig. 1 Spinal device sales in the United States. BMP = bone morphogenetic protein. (Reprinted, with …

The influence of antibiotics on the fatigue life of acrylic bone cement.

Aseptic loosening attributed to cement fracture and subsequent disruption of fixation interfaces remains a major long-term mode of failure of arthroplasties performed with cement1 (Fig. 1). The fracture strength of bone cement, especially in fatigue, is an important indicator of cement integrity and the potential for fixation failure. In current practice, orthopaedic surgeons may choose to utilize antibiotics in bone cement for prophylaxis or treatment of a known infection. However, the antibiotics, bone cement, and mixing method employed lead to variability in the quality of the end product2-5. To date, several United States orthopaedic manufacturers have received Food and Drug Administration (FDA) 510(k) clearance for use of their prepackaged antibiotic-loaded bone cement for the second stage of a two-stage revision after the initial infection has been eradicated. This availability provides a more uniform cement mix with known mechanical and elution characteristics at the level of 0.5 to 1.0 g of antibiotic per 40 g of polymer powder. In this study, we evaluated the influence of antibiotic inclusion on the porosity, strength, and fatigue life of six contemporary bone cements. Six surgical quality and commercially available surgical bone cements were evaluated: Endurance and Endurance Gentamicin (DePuy Orthopaedics, Warsaw, Indiana), Surgical Simplex P and Simplex P with Tobramycin (Stryker Orthopaedics, Mahwah, New Jersey), and VersaBond and VersaBond AB with Gentamicin (Smith and Nephew, Memphis, Tennessee). Ten 1.2-g doses of Tobramycin for Injection USP (United States Pharmacopeia) (Pharma-Tek, Huntington, New York), which is the base quantity supplied to the operating theater when antibiotics are hand-mixed with bone cement, were acquired. Three vacuum-mixing systems (Figs. 2-A, 2-B, and 2-C)—the Cemvac (DePuy Orthopaedics), the ACM (Advanced Cement Mixing System; Stryker), and the Vortex (Smith and Nephew)—were acquired directly from their manufacturers. Nine study samples were created: hand-mixed …

Do thin acetabular shells increase the disassociation risk of ceramic liners

Introduction In total hip arthroplasty, the procedural standard for the fixation of metallic acetabular shells is underreaming and impaction. Recently, concerns have arisen regarding achieving and maintaining secure ceramic liners when thin shells are deformed during normal insertion. Failure to achieve an adequate ceramic taper lock has been associated clinically with disassembly and liner fracture. Additionally, this phenomenon has been hypothesized to be a possible cause of articular squeaking, leading to revision in a small number of patients. This study evaluates the influence of shell deformation on the locking-mechanism integrity of contemporary modular acetabular designs that employ ceramic liners.

Clinical retrieval and simulator comparison of an investigational cervical disc replacement: an a priori requirement.

IntroductionThe evolvement of cervical and lumbar disc replacement designs as alternatives to spinal fusion has resulted in a substantial number of ongoing United States Food and Drug Administration (FDA)-sponsored clinical trials. While these trials seek to establish “safety and effectiveness,” the