Michael J. Pappas

Twenty-year evaluation of meniscal bearing and rotating platform knee replacements.

Clinical results of the initial cemented and cementless series of 373 New Jersey Low Contact Stress total knee replacements in 282 patients surviving at least 10 years were analyzed using a strict knee scoring scale. The study showed excellent, good, fair, or poor results in 68.1%, 29.8%, 2.1%, or 0% of primary posterior cruciateretaining meniscal bearing knee replacements, 46.7%, 53.3%, 0%, or 0% results in primary cemented rotating platform knee replacements, and 68.1%, 29.8%, 2.1%, or 0% results in primary cementless rotating platform knee replacements, respectively. Radiographic evaluation at minimum 10-year followup showed stable fixation of all components, no gross migration but significant osteolysis requiring bearing exchange and bone grafting in three cementless rotating platform knee replacements (1.8%) in three patients who underwent previous surgeries at an average of 10.2 years from the index surgery. Survivorship of the patients who underwent primary cementless posterior cruciate-retaining meniscal bearing knee replacements with an end point of revision for any mechanical reason was 97.4% at 10 years and 83% at 16 years; using an end point of a poor clinical knee score the survivorship was 98.9% at 10 years and at 16 years. Survivorship of the patients who underwent primary cemented rotating platform knee replacements with end points of revision for any mechanical reason or a poor clinical knee score was 97.7% at 10 years and at 20 years. Survivorship of the patients who underwent cementless rotating platform knee replacements with end points of revision for any mechanical reason or a poor clinical knee score was 98.3% at 10 years and at 18 years.

New Jersey Low Contact Stress Total Ankle Replacement: Biomechanical Rationale and Review of 23 Cementless Cases

A congruent contact, unconstrained, multiaxial ankle replacement has been developed for use without cement. A talar onlay component with a trochlear surface and central fixation fin uses a cylindrical articulating axis that reproduces the lateral talar curvature. A tibial inlay component with a 7° anteriorly inclined short fixation stem uses a flat loading plate, recessed anatomically into the distal tibia to distribute tibial loads to the ankle joint. For both components, made of cast cobalt-chromium-molybdenum, a 275-μm pore-size, sintered-bead, porous coating is used to allow tissue ingrowth stabilization. A congruent ultrahigh molecular weight polyethylene bearing is inserted between the metallic implants. Its upper surface is flat, whereas its lower surface conforms to the trochlear surface, thereby providing unconstrained, sliding cylindrical motion with low contact stress on the bearing surfaces. Contact pressure and collateral ligaments maintain ankle stability during both static and dynamic loading conditions. Clinically, 23 total ankle arthroplasties were performed in 21 patients. The follow-up period ranged from 24 months to 64 months with a mean of 35.3 months. Diagnoses included rheumatoid arthritis, 6 patients (26.1%); osteoarthritis, 4 patients (17.4%); post-traumatic arthritis, 10 patients (43.5%); avascular necrosis of the talus, 2 patients (8.7%), and painful ankle fusion, 1 patient (4.3%). Pain was the primary reason for surgery in all cases. Postoperatively, 87% of ankles had no pain or, at most, mild pain. Postoperative complications included poor wound healing in four ankles, reflex sympathetic dystrophy in two ankles, deep infection in one ankle, and one bearing subluxation. No ankle replacements were removed and no fusions were performed for failed implants, although one bearing was exchanged without disrupting the metallic elements. In this report, the suggestion is made that total ankle arthroplasty may have an improved application in various arthritis disorders when used with biologic fixation and unconstrained mobile bearings.

Ten-Year Evaluation of Cementless Buechel-Pappas Meniscal Bearing Total Ankle Replacement

A porous-coated, cementless, congruent-contact, three-piece, meniscal-bearing total ankle replacement was developed and used clinically over a 2- to 10-year period for patients with disabling ankle arthritis. Polished titanium-nitride ceramic-coated Ti6Al4V tibial and talar components with a deep-sulcus trochlear groove and two lateral fixation fins for the talar onlay component were used. The ultra-high-molecular-weight polyethylene (UHMWPe) meniscal bearing congruently conformed to the flat upper tibial component surface and the deep sulcus and cylindrical geometry of the lower talar component surface. Fifty deep-sulcus (Buechel-Pappas™) total ankle replacements were implanted in 49 patients. Diagnoses were 8 osteoarthritis (16%), 7 rheumatoid arthritis (14%), 2 avascular necrosis (4%), and 33 post-traumatic arthritis (66%). Ages ranged from 26 to 71 years (mean 49 years). Clinical results using a strict ankle scoring system demonstrated good/excellent results in 88% of cases. Postoperative ankle motion ranged from 12° to 46° total arc (mean 28°), which was similar to the preoperative motion. Revision for malalignment was necessary in two cases (4%). Mechanical complications included one case of meniscal bearing wear (2%) in a patient with posttraumatic arthritis with component malalignment and one case of talar component subsidence (2%) in a patient with avascular necrosis of the talus. No tibial component loosening was seen. Cumulative survivorship using an end point of revision of any component for any reason was 93.5% at 10 years (confidence interval 61–100%).

Evaluation of contact stress in metal-backed patellar replacements. A predictor of survivorship.

Significant wear-through and dissociation failures of metal-backed, point and line contact patellar replacements are associated with excessive contact stresses on the polyethylene-bearing surfaces. Analytical mathematical contact-stress analysis was used to evaluate various patellar component geometries under loading conditions consistent with walking, stair descent, and deep knee bends, respectively. Typical point- and line-contact patellar-surface geometries exceeded the manufacturers recommended maximum permissible compressive stress level of 10 MPa by a factor greater than three, which also exceeds the yield stress of ultra-high, molecular-weight polyethylene. A metal-backed, rotating-bearing, area contact geometry patellar replacement maintained safe contact stress levels at less than half of the maximum permissible compressive stress level. These contact stress analyses predict early fatigue failure of all-polyethylene or metal-backed, point- and line-contact patellar replacements, while predicting long-term survival of area-contact, rotating-patellar replacements. Clinical evaluation of these various implants support the conclusions of these analyses. Contact stress analysis should precede any clinical use of patellar implants to avoid predictable failure mechanisms.

An Improved Direct Search Numerical Optimization Procedure.

Abstract An improved, nonlinear, constrained mathematical programming optimization algorithm is presented in this report. It couples a rotating coordinate pattern search with a feasible direction finding procedure used at points of pattern search termination. The procedure is compared with nineteen algorithms, including most of the popular methods, on ten test problems. These problems are such that the majority of codes failed to solve more than half of them. The new method proved superior to all others in the overall generality and efficiency rating, being the only one solving all problems. It was particularly effective on constrained problems where it was best in all rating categories.

MATHEMATICAL PROGRAMMING PROCEDURES FOR MIXED DISCRETE-CONTINUOUS DESIGN PROBLEMS

Abstract : Two mathematical programming procedures for treating nonlinear problems involving mixed variables are presented. One involves a relatively simple concept. First an optimum is located treating all variables as continuous. Adjacent discrete points are then evaluated in order of increasing distance from the all-continuous optimum, each evaluation requiring an optimization of the continuous variables, if any, until a satisfactory design is found. The other method utilizes an optimal discrete search to locate the optimum. These procedures are applied to the minimum weight design of stiffened, cylindrical shells where they prove to be effective. (Author)

Structural synthesis of frame reinforced, submersible, circular, cylindrical hulls☆

Abstract This paper describes a mathematical programming procedure for the automated optimal structural synthesis of frame stiffened, cylindrical shells. For a specified set of design parameters such as external pressure, shell radius and length and material properties, the method generates those values of the design variables that produce a minimum weight design. The skin, frame web and frame flange thicknesses and the flange width are treated as continuous variables. Frame spacing is considered a discrete variable. Constraint equations control local and general shell and frame instability and yield. Limits may be placed on the variable values, and certain geometric or space constraints can be applied. The mixed (continuous and discrete nonlinear programming problem is solved by a combination of a discrete ‘Golden Search’ for the optimal number of frames and the ‘Direct Search Design Algorithm’ which provides the optimum values of the continuous variables.

Extended capability for automated design of frame-stiffened, submersible, cylindrical shells

Abstract This paper presents a procedure and computer program for the minimum weight design of circular, cylindrical, ‘T’ frame (ring) reinforced, submersible shells where all metal thicknesses may be confined to specified gage thickness values. Using the designer specified parameters defining shell radius shell length, eccentricity, operating depth, design factors of safety, construction materials properties and when used, the specified gage thickness values, the program will generate those values of skin thickness stifiener web and flange thicknesses, stiffener web depth and flange width, and if desired, stiffener spacing that will produce the smallest shell weight to liquid weight displaced ratio. Experience with the program has demonstrated that there is usually little weight penalty associated with the use of discrete metal thickness values when the stiffener spacing can be optimized. This weight penalty can, however, be significant where the number of stiffeners is held fixed.

Failure Modes of Current Total Ankle Replacement Systems

Methodology for evaluation of total ankle replacements is described. Fusion and its problems are discussed as are those of total ankle joint replacement. Fusion is an imperfect solution because it reduces ankle functionality and has significant complications. Early fixed-bearing total ankles were long-term failures and abandoned. Currently available fixed-bearing ankles have proved inferior to fusion or are equivalent to earlier devices. Only mobile-bearing devices have been shown reasonably safe and effective. One such device, the STAR, has been approved by the Food and Drug Administration after a rigorous controlled clinical trial and is available for use in the United States.

Improved synthesis capability for “T” ring-stiffened cylindrical shells under hydrostatic pressure

Abstract The new “Direct Search-Feasible Direction” (DSFD) nonlinear mathematical programming optimization algorithm is applied to the design of stiffened submersible shells. An automated design capability for this problem (SBSHL6) is described wherein the program will generate the least weight design by locating the optimal, or near optimal, values of skin thickness, web thickness and height, flange thickness and width, and stiffener spacing given the design parameters such as shell size, immersion pressure, shell eccentricity, materials properties, and minimum natural frequency. Constraint equations control, general, panel (between stiffener), web, and flange instability, skin and stiffener yielding, and minimum natural frequency. The DSFD procedure appears capable of reliably locating optimal designs whereas earlier attempts in investigations using other optimization methods, including the popular SUMT procedure, failed to provide optimal solutions to the same problem. This and an earlier detailed comparison study strongly suggest that SUMT is not a reliable procedure for structural optimization while DSFD seems to provide reasonably reliable performance. Designs generated by SBSHL6 are presented and compared with those of the earlier studies. The results of a series of synthesis runs from widely separated starting points are also presented. The designs developed by SBSHL6 are substantially lighter than those reported earlier. The multipath runs for each set of parameters studied all converged to similar designs of essentially identical weights demonstrating program reliability.