Direct measurement of resultant forces in the anterior cruciate ligament. An in vitro study performed with a new experimental technique

A new technique was used to measure the resultant forces in the anterior cruciate ligament during a series of loading experiments on seventeen fresh-frozen cadaver specimens. The base of the ligament's tibial attachment was mechanically isolated with a coring cutter, and a specially designed load-transducer was fixed to the bone-plug that contained the ligament's tibial insertion so that the resultant forces were directly measured by the load-cell. Although the magnitudes of values for forces varied considerably between specimens for a given test condition, the patterns of loading with respect to direction of loading and the angle of flexion of the knee were remarkably consistent. Passive extension of the knee generated forces in the ligament only during the last 10 degrees of extension; at 5 degrees of hyperextension, the forces ranged from fifty to 240 newtons (mean, 118 newtons). When a 200-newton pull of the quadriceps tendon was applied to extend a knee slowly against tibial resistance, however, the force in the ligament increased at all angles of flexion of the knee. Internal tibial torque always generated greater forces in the ligament than did external tibial torque; higher forces were recorded as the knee was extended. The greatest forces (133 to 370 newtons) were generated when ten newton-meters of internal tibial torque was applied to a hyperextended knee. Fifteen newton-meters of applied varus moment generated forces of ninety-four to 177 newtons at full extension; fifteen newton-meters of applied valgus moment generated a mean force of fifty-six newtons, which remained unchanged with flexion of the knee. The force during straight anterior translation of the tibia was approximately equal to the anterior force applied to the tibia. The application of 925 newtons of tibiofemoral contact force reduced the mean force in the ligament that was generated by 200 newtons of anterior pull on the tibia by 36 per cent at full extension and 46 per cent at 20 degrees of flexion.     

Strength of initial mechanical fixation of screw ring acetabular components

This study was conducted to determine the effects of design on the initial fixation of several types of screw-ring acetabular components. The components were tested in polyurethane foam to assess relative screw fixation strengths with a consistent material. Embalmed pelves from anatomic specimens were used to conduct paired tests between designs that showed large differences in insertional torque to failure in foam. The quality of the initial fixation in foam was found to be dependent on the design features of the components. Components with widely spaced, deep threads, and minimal thread interruptions offered the strongest initial fixation in foam. Tests in bone revealed a wide range of fixation strengths reflecting the variability in bone quality. No differences in fixation strength attributable to component design were observed in bone. When the insertional failure torque was greater than 60 N.m, one-half of the pelves fractured, and these fractures occurred with all designs. At failure torques less than 60 N.m, failure was predominantly due to thread strippage of the screw, with only two of 20 specimens experiencing pelvic fracture.     

Porous surface replacement of the hip with chamfered-cylinder component

One hundred porous surface replacements (PSR) were performed in 92 patients (63 men and 29 women) with a mean age of 53 (range 17-76). Follow-up times range from 1 to 4 years, with 48 patients having a follow-up of at least 2 years. Preoperative diagnoses were osteoarthritis (OA) 63, osteonecrosis (ON) 13, dysplasia 9, rheumatoid-ankylosing spondylitis 6, and other 9. Seventeen hips had metal-backed acrylic-fixed THARIES acetabular sockets, nine hips had a porous cobalt chrome hemispheric beaded acetabular component with adjuvant fixation screws and externally protruding screw hubs, and 74 hips had a porous chamfered cylinder-design acetabulum. Pain relief had been immediate and more complete than with acrylic-fixed or biologic-ingrowth stem-type replacement with comparable walking and function improvements. There have been no major systemic complications, sepsis, or loosening. There have been two transient peroneal nerve palsies and three trochanteric fibrous unions. There have been three reoperations, one for subluxation, one for "metalosis" due to mesh pad loosening, and one femoral neck fracture. Examination of one removed femoral surface component which has been histologically sectioned revealed excellent (90%) bone in-growth. Circumferential progressive radiolucencies developed at the bone-cement interface by 1 year in all of the 17 acrylic-fixed acetabular components. Reaming or seating defects were noted in 25% of the ingrowth components on postoperative radiographs. Radiographic analysis of immediate postoperative films of the chamfered cylinder design acetabular components frequently demonstrated bone-component interface radiolucencies which represented component seating defects. These initial interface radiolucencies became progressively more narrow over the first six months postoperatively suggesting "healing" of the reamed bone-component interface with trabecular bone around the chamfered cylinder acetabular components. Partial healing of initial interface voids with residual narrow radiolucencies were typical of the nine hemispheric-design acetabula with adjuvant screws and screw hubs. This new porous surface replacement (PSR) of the hip using porous ingrowth fixation avoids the major disadvantages of acrylic-fixed SR: excessive acetabular reaming and difficulty with acetabular revision. (When conversion to stem-type replacement is necessary the modular polyethylene socket liner can be exchanged.) The PSR has the prospect of enhanced fixation and improved longterm durability.     

Continued growth of the proximal part of the tibia after prosthetic reconstruction of the skeletally immature knee. Estimation of the minimum growth force in vivo in humans.

We studied five skeletally immature patients who had a cemented endoprosthetic replacement involving the proximal part of the tibia because of a malignant tumor. In each patient, the cement-column fractured, allowing additional physeal growth. With plain radiographs and scanograms, we determined the cross-sectional areas of the physes, the cement-mantle, and the tibial component. Using the known tensile strength of polymethylmethacrylate cement, we then calculated the minimum force that the growth plates must have overcome to fracture the cement. This averaged 584 newtons per square centimeter. This observation of continued tibial growth after partial physeal ablation with a cemented prosthesis in skeletally immature patients presented a unique opportunity to estimate the force generated in the human physis during growth.     

Computerized anatomy instruction

A self-paced gross anatomy teaching package is under development using computer-generated anatomic images controlled by branching instructional software on a personal computer. The package is designed to overcome several problems encountered in the traditional study of anatomy. The capabilities, strengths, and weaknesses of computer-assisted instruction are highlighted.     

The effects of pressurization on fracture swelling and joint stiffness in the rabbit hind limb

Long bone fractures frequently cause limb swelling and joint stiffness, and the two effects are generally assumed to be related, an assumption long held but rarely tested. The authors investigated limb swelling and joint stiffness in rabbits with experimental 28-day healing fractures. Eleven control rabbits were treated with bilateral hind limb skeletal fixation and unilateral distal tibia fractures. Subsequent limb swelling occurred bilaterally, but it was significantly greater on the side fractured. All fractures healed. Ankles in both fractured and unfractured limbs became significantly stiffer, compared with preinjury values, but ankles in the fractured limbs became significantly stiffer than those in the unfractured control limbs. Air pressure of 10 mmHg above atmospheric pressure was applied for four days to newly fractured limbs in 11 additional rabbits. Pressurization reduced swelling but did not significantly affect ankle stiffness. In general, reduction in postfracture swelling by application of external pressure did not significantly affect adjacent joint stiffness.     

The effects of two nonsteroidal antiinflammatory drugs on limb swelling, joint stiffness, and bone torsional strength following fracture in a rabbit model

Using a rabbit distal tibia fracture model treated by immobilization for three weeks, the effects of two nonsteroidal antiinflammatory drugs on postfracture limb swelling, joint stiffness, and torsional bone strength were examined. Limb swelling in rabbits treated with low-dose piroxicam was reduced by 39%, and high-dose piroxicam reduced limb swelling by 86%. Flunixin, tested at one dose, reduced swelling by 53%. Neither piroxicam nor flunixin affected ankle stiffness or tibia torsional strength.     

The Dana total shoulder arthroplasty

Fifty-six Dana unconstrained total shoulder arthroplasties in forty-seven patients were followed for a minimum of two years. The preoperative diagnoses included osteoarthritis, rheumatoid arthritis, traumatic arthritis, avascular necrosis, failed hemiarthroplasty, and failed total shoulder arthroplasty of another design. For all of the patients, the average rating for pain improved from 3 points preoperatively to 8 points postoperatively, and the average rating for function improved from 3 to 7 points. The ranges of abduction and of external rotation improved substantially in the shoulders that were affected by osteoarthritis, rheumatoid arthritis, or osteonecrosis. Ten patients were treated with a hooded glenoid component, designed to improve stability in shoulders in which the rotator cuff is deficient. In these shoulders, both the rating for pain and the rating for function improved, although the range of motion did not. Complications that required revision of the arthroplasty developed in five shoulders in which a regular component had been implanted and in two that had a hooded glenoid component. A radiolucent line developed at the bone-cement interface of the glenoid component in fifty-three shoulders, but only two revisions were done for loosening of the glenoid component.     

Ultra-high-molecular weight polyethylene wear: an in vitro comparison of acetabular metal types and polished surfaces.

The generation of debris from the wear of ultra-high-molecular weight polyethylene (UHMWPE) is a well-recognized factor in the development of osteolysis and the long-term failure of total joint arthroplasties. Wear between the articulation of the femoral head and the polyethylene has been recognized for many years, but more recently, both retrieval and in vitro studies have demonstrated that convex surface wear or backside wear also occurs and may be of significance. Currently, modular acetabular components are being designed with polished surfaces, fewer screw holes, various polyethylene locking mechanisms, and stiffer metal alloys in an attempt to reduce backside wear. The purpose of this study was to determine if differences existed in UHMWPE wear based on the metal alloy used and the surface finish in modular acetabular components. Sixteen components in 4 groups were subjected to 10 million gait cycles using an in vitro joint simulator. All components used 28-mm cobalt chrome femoral heads on cobalt chrome tapered stems. The 4 groups differed only in the type of metal backing and type of interior finished surface: polished cobalt chrome, unpolished cobalt chrome, polished titanium, and unpolished titanium. UHMWPE changes were examined in terms of articular (concave) surface wear, backside (convex) surface wear, and frictional torque. The overall linear and volumetric wear rates were 1.05 mm/10 million cycles and 325 mm3/10 million cycles. No significant differences in linear and volumetric wear rates were detected between the cobalt chrome and titanium acetabular components. Surface finish did not influence wear rates. In terms of backside wear, all specimens in the 4 groups demonstrated total loss of all sputtered gold with the exception of those areas extruded through the screw holes. Extrusion through the screw holes was on the order of 0.0004 inch for all groups, and no significant difference was seen among the groups for this parameter. The measurements of articular frictional torque demonstrated a significant difference among the polished and unpolished cobalt chrome components (17.3 N x m vs 11.5 N x m; P = .0039, 2-way analysis of variance, Student's Newman Keuls method). Some designs in modular acetabular components have favored stiffer alloys, such as cobalt chrome, with polished concave surfaces to decrease wear on both the concave and the convex surfaces. In this study, there was no significant difference in wear rates noted between cobalt chrome and titanium acetabular components, and polishing of the components had no appreciable affect in reducing backside wear.