Stephen M. Belkoff

Immediate weight-bearing after treatment of a comminuted fracture of the femoral shaft with a statically locked intramedullary nail

BACKGROUNDnThe purpose of this two-part investigation was to test the feasibility, safety, and efficacy of immediate weight-bearing after treatment of fractures of the shaft of the femur with a statically locked intramedullary nail.nnnMETHODSnIn the first part of the investigation, a biomechanical study was performed to determine the fatigue strength of eleven different statically locked intramedullary nail constructs. Segmentally comminuted midisthmal fractures were simulated with use of sections of polyvinyl chloride pipe; each construct was cyclically loaded in compression with use of physiologically relevant loads in a materials testing machine at eight hertz. The fatigue tests were conducted according to the so-called staircase method, and the construct was considered to have run out (exceeded its anticipated service life) if it had not failed after 500,000 cycles. In the second part of the study, a clinical investigation of immediate weight-bearing after treatment of comminuted fractures of the femoral shaft with a Russell-Taylor (RT-2) construct was performed. Complete follow-up data were available for twenty-eight of the thirty-five patients (thirty-six fractures) entered into the study.nnnRESULTSnIn Part I of the study, two constructs, a statically locked twelve-millimeter-diameter Russell-Taylor femoral nail with two distal locking screws (RT-2) and a statically locked twelve-millimeter-diameter Zimmer femoral nail with two distal locking screws (Z-2), had significantly higher mean fatigue strengths (2171 and 2113 newtons, respectively) than all other constructs tested (p<0.001), but the strengths of these two constructs were not significantly different from each other. Constructs with only one distal locking screw demonstrated significantly lower (p<0.05) fatigue strengths than the two-screw constructs. These results suggest that full weight-bearing during the weeks immediately after insertion of the nail may be possible, even for patients who have a comminuted fracture of the femoral shaft. In Part II of the study, twenty-six of the twenty-eight patients were bearing full weight on the fractured limb or limbs at the six-week follow-up visit. All fractures united; only one of these needed an additional procedure (the removal of the screws five months after the insertion of the nail) to stimulate union. No loss of fixation, such as back-out or breakage of a locking screw or breakage or bending of the intramedullary nail, occurred.nnnCONCLUSIONSnWe concluded from this two-part investigation that immediate weight-bearing after stabilization of a comminuted fracture of the femoral shaft with a statically locked intramedullary nail is safe when the construct has a relatively high fatigue strength. Immediate weight-bearing after stabilization of a fracture of the femoral shaft permits patients who have multiple fractures of the extremity to walk and to participate in physical therapy earlier, possibly decreasing the duration of the hospital stay or reducing the need for prolonged rehabilitation on an inpatient basis.

The effect of monomer-to-powder ratio on the material properties of cranioplastic

Percutaneous vertebroplasty consists of injecting polymethylmethacrylate cement into the cancellous bone of vertebral bodies for the treatment of various lesions of the spine, including osteoporotic compression fractures. Clinicians practicing vertebroplasty commonly alter the mixture of monomer-to-powder recommended by the manufacturer in an effort to decrease viscosity and increase the working time. The purpose of the current study was to measure the effect that varying the monomer-to-powder ratio has on the compressive material properties of a cement (Cranioplastic) commonly used in vertebroplasty. Cylindrical specimens were prepared varying a monomer-to-polymer ratio of 0.40 to 1.07 ml/g and tested per the American Society for Testing and Materials standard F451. Specimens prepared at 0.53 mL/g, which is near the manufacturers recommended monomer-to-polymer mixture of 0.57 mL/g, exhibited the greatest mean values for ultimate compressive stress, yield stress, and elastic modulus. Specimens prepared at higher or lower ratios exhibited diminished strength, in some cases by as much as 24%. Although altering the monomer-to-powder ratio affects the cements material properties, it is as yet unknown if the decrease is clinically significant.

Ligamentous Restraints of the Second Tarsometatarsal Joint: A Biomechanical Evaluation:

Ligamentous injury of the tarsometatarsal joint complex is an uncommon, but disabling condition that frequently occurs in elite athletes. There are few options for managing these injuries, in part because the relative mechanical contribution of the ligaments of the tarsometatarsal joint is unknown, complicating decisions regarding which ligaments need reconstruction. In the current study, strength and stiffness of the dorsal, plantar, and Lisfranc ligaments of 20 paired cadaver feet were measured and compared. The plantar and Lisfranc ligaments were significantly stiffer and stronger than the dorsal ligament, and the Lisfranc ligament was significantly stronger and stiffer than the plantar ligament.

A biomechanical evaluation of the tibiofibular and tibiotalar ligaments of the ankle.

The purpose of this ex vivo biomechanical study was to determine the strength and stiffness of the anterior and posterior syndesmotic tibiofibular ligaments and the posterior tibiotalar component of the deltoid ligament. Injuries to these ligaments are a prevalent clinical problem, yet little is known about their mechanical behavior. Ten fresh-frozen cadaver lower extremities (average age at death, 72 ± 8 years) were harvested. The anterior and posterior tibiofibular ligaments and the posterior tibiotalar component of the deltoid were isolated and prepared as bone–ligament–bone complexes for tensile testing to determine strength, stiffness, and mode of failure. The posterior tibiofibular ligament exhibited greater strength, but not significantly so (p <.05), than the anterior tibiofibular ligament and the posterior tibiotalar component of the deltoid ligament. There were no significant differences in stiffness between the three ligaments tested. The dominant mode of failure for the anterior tibiofibular ligament was ligament substance rupture, primarily near its fibular insertion, whereas the failure modes of the posterior tibiofibular ligament were evenly split between substance ruptures and fibular avulsions. The posterior tibiotalar component of the deltoid ligament ruptured most often near the talar insertion. The tibiofibular ligaments showed greater strength than the lateral collateral and deltoid ligaments, as mentioned in literature. The greater strength of the tibiofibular ligaments relative to the lateral collateral and deltoid ligaments suggests that these ligaments play an important role in ankle constraint.

Biomechanical evaluation of transverse acetabular fracture fixation

The purpose of this two-part biomechanical study was to evaluate various fixation methods for transverse acetabular fractures in a synthetic pelvic model. In Part 1, 40 transverse acetabular fractures were repaired with anterior column plating using 10-hole curved reconstructions plates with one of four screw configurations to evaluate the effect of screw placement and number on fracture fixation stiffness. In Part 2, 36 transverse acetabular fractures were repaired with one of six fixation methods using combinations of contoured plates and column screws to stabilize the anterior column, the posterior column, or both. Each repaired acetabulum was loaded via a hemiarthroplasty in a direction consistent with stance phase. Fixation stiffness was measured from the force-displacement curve for each construct. In Part 1, there was no significant difference in fixation stiffness afforded by any of the constructs. However, the stiffest construct resulted from two screws on each side of the fracture site: one placed as close to the fracture site as allowed (one empty screw hole adjacent to the fracture) and the second at the end of the plate. In Part 2, the constructs that concomitantly stabilized anterior and posterior columns were significantly stiffer than were those addressing either the anterior or posterior column alone, regardless of the number of plates applied. The stiffest construct combined a posterior column plate with an anterior column screw. Because no significant change in stiffness occurred with the addition of a third set of screws, two screws on each side of the fracture site appear to provide sufficient stability with acetabular plating. Concurrent fixation of anterior and posterior columns of transverse acetabular fractures provides the greatest resistance to postoperative loss of reduction in this model.

Collagen fibril D-period may change as a function of strain and location in ligament

The purpose of this study was to determine if the characteristic banding pattern (D-period) of collagen fibrils from rabbit medial collateral ligaments changes as a function of gross ligament strain and, if so, whether the changes are location dependent (insertion versus midsubstance). Femur-medial collateral ligament-tibia complexes were strained to 0, 8, or 12% and immediately chemically fixed in situ. Samples were taken from the medial collateral ligament midsubstance and bony insertions, and prepared for and observed under a transmission electron microscope. D-period length was measured and found to increase (albeit not significantly so, p=0. 1) as a function of gross strain for samples obtained from the insertion sites but not for samples obtained from the ligament midsubstance. Results suggested that ligament strains are inhomogeneous at the ultrastructural level.

The Effects of Tibial Malrotation on the Biomechanics of the Tibiotalar Joint

The effects of tibial malrotation on the biomechanics of the tibiotalar joint were studied using a cadaveric model loaded in an Instron 8521 materials testing device and a TEKScan I-Scan thin-film resistive ink pressure measuring system. Testing of 23 legs was performed using rotational conditions of 10 and 20° internal and external rotation as well as neutral rotation. All rotational conditions were found to decrease joint contact area. Peak pressures were significantly greater with 20° internal rotation as well as 20° external rotation. Total load across the joint was significantly lower for both 10 and 20° of external rotation. In conclusion, rotational deformity across the tibiotalar joint results in significant alteration of overall joint biomechanics and should be minimized whenever possible.

Biomechanical comparison of fixation of type-I fractures of the lateral tibial plateau: IS THE ANTIGLIDE SCREW EFFECTIVE?

Type-I fractures of the lateral tibial plateau were simulated by osteotomy in 18 pairs of unembalmed cadaver tibiae. One fracture of each pair was fixed with two lag screws whereas the contralateral site was stabilised with three lag screws, or two lag screws plus an antiglide screw. The lateral plateau was displaced downwards using a servohydraulic materials testing machine and the resulting force and articular surface gap were recorded. Yield load was defined as the maximum load needed to create a 2.0 mm articular offset at the fracture line. The yield loads of the three-lag-screw (307 +/- 240 N) and antiglide constructs (342 +/- 249 N) were not significantly different from their two-screw control constructs (231 +/- 227 and 289 +/- 245 N, respectively). We concluded that adding an antiglide screw or a third lag screw did not provide any biomechanical advantage in stabilising these fractures.

Mechanical Evaluation of a Soft Cast Material

In this study, the structural and material properties of a new semi-rigid material, Scotchcast SoftCast (SCS), were compared to the properties of two rigid materials, plaster of paris (POP) and Scotchcast Plus (SCP). Cylinders and flat beams made from 4, 6, 8, and 10 layers of each casting material were tested in three-point bending and diametrical compression. Initial stiffness and yield force values of SCS casts were significantly lower than for casts of SCP and POP made of the same number of layers. Casts made from SCS may be indicated for non-rigid applications, but not where rigid immobilization is required.

Effect of modular head seating on the cement-stem interface strength of femoral prostheses.

This study tested the hypothesis that modular head seating in situ reduces the cement-stem interfacial strength. Femoral prostheses were implanted in eight pairs of cadaveric femurs; one femur of each pair received a stem for which the modular head was seated by dropping a weight from a given height (treated); the contralateral femur received a nonseated stem (control). Transverse sections were cut from four standardized locations on each implanted femur, and the yield shear stress and ultimate shear stress of the interface were determined from push-out tests. There was no significant difference in cement-stem interfacial strength between seated and nonseated prostheses. These results suggest that seating modular heads in situ has no deleterious effect on the acute interfacial strength of cemented femoral implants.