The effects of partial and total interosseous membrane transection on load sharing in the cadaver forearm.

This study was performed to examine the effects of partial and total transection of the interosseous membrane (IOM) on load transfer in the forearm. Twenty fresh frozen forearms were instrumented with custom designed load cells placed in the proximal radius and distal ulna. Simultaneous measurements of load cell forces, radial head displacement relative to the capitellum, and local tension within the central band of the IOM were made as the wrist was loaded to 134 N with the forearm at 90 degrees of elbow flexion and in neutral pronation supination. For valgus elbow alignment (radial head contacting the capitellum), mean force carried by the distal ulna was 7.1% of the applied wrist force and mean force transferred from radius to ulna through the IOM was 4.4%. For varus elbow alignment (mean 2.0 mm gap between the radial head and capitellum), mean distal ulna force was 28% and mean IOM force was 51%. Section of the proximal and distal one-thirds of the IOM had no significant effect upon mean distal ulnar force or mean IOM force. Total IOM section significantly increased mean distal ulnar force for varus elbow alignment in all wrist positions tested. The mean level of applied wrist force necessary to close the varus gap (89 N) decreased significantly after both partial IOM section (71 N) and total IOM section (25 N). The IOM became loaded only when the radius displaced proximally relative to the ulna, closing the gap between the radius and capitellum. As the radius displaced proximally, the wrist becomes increasingly ulnar positive, which in turn leads to direct loading of the distal ulna. This shift of force to the distal ulna could present clinically as ulnar sided wrist pain or as ulnar impaction after IOM injury.

Effects of radial head excision and distal radial shortening on load-sharing in cadaver forearms.

BACKGROUND: The present study was performed to measure changes in radioulnar load-sharing in the cadaveric forearm following two orthopaedic surgical procedures that often have varying results: radial head excision and distal radial shortening. A better understanding of the biomechanical consequences of those procedures could aid surgeons in obtaining a more satisfactory clinical outcome. METHODS: Miniature load-cells were inserted into the proximal part of the radius and the distal part of the ulna in twenty fresh-frozen cadaveric forearms. Load-cell forces, radial head displacement relative to the capitellum, and local tension within the central band of the interosseous membrane were measured simultaneously as the wrist was loaded to 133.5 N in neutral pronation-supination and neutral radioulnar deviation. Testing was repeated after incremental distal radial shortening and after removal of the radial head. RESULTS: With the elbow flexed to 90 degrees and in valgus alignment (the radial head in contact with the capitellum), the mean force in the distal part of the ulna was 7.1% of the applied wrist force and the mean force in the interosseous membrane was 4.0%. With the elbow in varus alignment (a mean initial gap of 1.97 mm between the radial head and the capitellum), the respective mean values were 27.9% and 51.2%. After excision of the radial head, the mean force in the distal part of the ulna increased to 42.4% of the applied wrist force and the mean force in the interosseous membrane increased to 58.8%, in both varus and valgus elbow alignment. The mean distal ulnar force increased with progressive distal radial shortening in both varus and valgus elbow alignment; after 6 mm of radial shortening, the distal ulnar force averaged 92.4% (in varus alignment) and 60.9% (in valgus alignment). Equal distal load-sharing between the radius and ulna occurred after approximately 5 mm of radial shortening with the elbow in valgus alignment and after approximately 2 mm of radial shortening with the elbow in varus alignment. In valgus alignment, the force in the interosseous membrane was negligible after all degrees of radial shortening; in varus alignment, the mean force in the interosseous membrane decreased from 51.2% (0 mm of distal radial shortening) to 0% (6 mm of distal radial shortening) because of progressive slackening of the interosseous membrane. CONCLUSIONS: Radial head excision shifted the applied wrist force that normally would be transmitted to the elbow, through radial head-capitellar contact, to the interosseous membrane. The resulting proximal radial displacement created an ulnar-positive wrist and increased distal ulnar loading. Radial shortening and ulnar lengthening procedures have been designed to shift the applied wrist force from the distal part of the radius to the distal part of the ulna; it is commonly assumed that these procedures have equivalent biomechanical effects. We found that radial shortening resulted in slackening of the interosseous membrane, thereby negating its ability to transmit load through the forearm. Slackening of the interosseous membrane would not be expected with distal ulnar lengthening procedures. CLINICAL RELEVANCE: When the radial head has been fractured or excised, the mechanical status of the interosseous membrane is critical to the load-sharing process. If the interosseous membrane remains intact, distal ulnar loads will be limited to less than half of the applied wrist force; if the interosseous membrane has been damaged, nearly the entire applied wrist force will be shifted to the ulna. The amount of radial shortening or ulnar lengthening performed at the time of surgery during joint-leveling procedures has been largely empirical. We found that distal ulnar load increased by approximately 10% for each millimeter of radial shortening.

The Frank Stinchfield Award. Influence of cement technique on the interface strength of femoral components.

The optimal surface finish for polymethylmethacrylate cemented femoral components remains controversial. Concerns about early debonding of the prosthesis-cement interface have led surgeons to use roughened surfaces to enhance the cement-prosthesis bond. However, loosening of roughened stems is associated with the generation of excessive wear debris. The purpose of the current study was to determine whether the time to cementation influenced the cement-prosthesis bond of four roughened cobalt chrome surfaces (60 grit-blasted, 10 grit-blasted, 10 grit-blasted with polymethylmethacrylate precoating, glass bead-blasted) and one polished cobalt chrome surface. Fixation strength was assessed using mechanical pushout and tensile testing. Roughened and polymethylmethacrylate precoated surfaces had significantly greater tensile and shear strengths at early cementation times compared with polished surfaces. However, roughened components had significant decreases in tensile and shear strengths as cementation time increased from 2 to 4 minutes and 2 to 6 minutes. In contrast, tensile and shear strengths for the polished surface were significantly lower than for the roughened surfaces and did not change with longer cementation times. When using a roughened or precoated cemented femoral component, the surgeon should consider cementing earlier with wetter cement to maximize the cement-prosthesis bond. When implanting a polished femoral component, it is preferable that the cement is doughy, because the cement-prosthesis bond is not influenced by the wetness of the cement and it is easier to maintain the orientation of the femoral 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.

Public health nurses' responses to domestic violence: a report from the Enhanced Domestic Abuse Intervention Project.

Public health nurses (PHNs) can play an important role in the detection of domestic violence. This study examines whether the introduction of a domestic violence assessment protocol by public health nurses in a maternal and child health visiting program increases the identification and referral rates of women experiencing domestic violence. Data collected from case files during the baseline year prior to the initiation of the protocol were compared to case file information after the protocol had been implemented. When the protocol was used, there was a higher rate of identification, although the difference was not statistically significant. Significantly more women, however, were provided with information about domestic violence resources after the protocol was in place, and significantly more women were referred to services in the second year after the protocol had been implemented. This study provides support for the use of a domestic violence protocol to improve the public health nursing response to domestic violence.

Combined knee loading states that generate high anterior cruciate ligament forces.

Injuries to the anterior cruciate ligament frequently occur under combined mechanisms of knee loading. This in vitro study was designed to measure levels of ligament force under dual combinations of individual loading states and to determine which combinations generated high force. Resultant force was recorded as the knee was extended passively from 90 degrees of flexion to 5 degrees of hyperextension under constant tibial loadings. The individual loading states were 100 N of anterior tibial force, 10 Nm of varus and valgus moment, and 10 Nm of internal and external tibial torque. Straight anterior tibial force was the most direct loading mechanisms; the mean ligament force was approximately equal to applied anterior tibial force near 30 degrees of flexion and to 150% of applied tibial force at full extension. The addition of internal tibial torque to a knee loaded by anterior tibial force produced dramatic increases of force at full extension and hyperextension. This loading combination produced the highest ligament forces recorded in the study and is the most dangerous in terms of potential injury to the ligament. In direct contrast, the addition of external tibial torque to a knee loaded by anterior tibial force decreased the force dramatically for flexed positions of the knee; at close to 90 degrees of flexion, the anterior cruciate ligament became completely unloaded. The addition of varus moment to a knee loaded by anterior tibial force increased the force in extension and hyperextension, whereas the addition of valgus moment increased the force at flexed positions. These states of combined loading also could present an increased risk for injury. Internal tibial torque is an important loading mechanism of the anterior cruciate ligament for an extended knee. The overall risk of injury to the ligament from varus or valgus moment applied in combination with internal tibial torque is similar to the risk from internal tibial torque alone. External tibial torque was a relatively unimportant mechanism for generating anterior cruciate ligament force.