Allan F. Tencer

Factors Affecting the Pullout Strength of Cancellous Bone Screws

Screws placed into cancellous bone in orthopedic surgical applications, such as fixation of fractures of the femoral neck or the lumbar spine, can be subjected to high loads. Screw pullout is a possibility, especially if low density osteoporotic bone is encountered. The overall goal of this study was to determine how screw thread geometry, tapping, and cannulation affect the holding power of screws in cancellous bone and determine whether current designs achieve maximum purchase strength. Twelve types of commercially available cannulated and noncannulated cancellous bone screws were tested for pullout strength in rigid unicellular polyurethane foams of apparent densities and shear strengths within the range reported for human cancellous bone. The experimentally derived pullout strength was compared to a predicted shear failure force of the internal threads formed in the polyurethane foam. Screws embedded in porous materials pullout by shearing the internal threads in the porous material. Experimental pullout force was highly correlated to the predicted shear failure force (slope = 1.05, R2 = 0.947) demonstrating that it is controlled by the major diameter of the screw, the length of engagement of the thread, the shear strength of the material into which the screw is embedded, and a thread shape factor (TSF) which accounts for screw thread depth and pitch. The average TSF for cannulated screws was 17 percent lower than that of noncannulated cancellous screws, and the pullout force was correspondingly less. Increasing the TSF, a result of decreasing thread pitch or increasing thread depth, increases screw purchase strength in porous materials. Tapping was found to reduce pullout force by an average of 8 percent compared with nontapped holes (p = 0.0001). Tapping in porous materials decreases screw pullout strength because the removal of material by the tap enlarges hole volume by an average of 27 percent, in effect decreasing the depth and shear area of the internal threads in the porous material.

Norian Srs Cement Augmentation in Hip Fracture Treatment: Laboratory and Initial Clinical Results

Bone quality, initial fracture displacement, severity of fracture comminution, accuracy of fracture reduction, and the placement of the internal fixation device are important factors that affect fixation stability. New high strength cements that are susceptible to remodeling and replacement for fracture fixation may lead to improved clinical outcome in the treatment of hip fractures. Norian SRS is an injectable, fast setting cement that cures in vivo to form an osteoconductive carbonated apatite of high compressive strength (55 MPa) with chemical and physical characteristics similar to the mineral phase of bone. It can be used as a space filling internal fixation device to facilitate the geometric reconstruction, load transfer, and healing of bone with defects and/or fractures in regions of cancellous bone. Furthermore, this cement can improve the mechanical holding strength of conventional fixation devices. Use of this material potentially could improve fracture stability, retain anatomy during fracture healing and improve hip function, thus achieving better clinical outcomes. In vivo animal studies have shown the materials biocompatibility, and cadaveric studies have shown the biomechanical advantage of its use in hip fractures. Initial clinical experience (in 52 femoral neck fractures and 39 intertrochanteric fractures) showed the potential clinical use of this innovative cement in the treatment of hip fractures.

Two-, four-, and six-strand zone II flexor tendon repairs: An in situ biomechanical comparison using a cadaver model

A dynamic in vitro model of zone II flexor tendon repair was used to compare gliding resistance, gap formation, and ultimate strength of the 2-, 4-, and 6-strand repair techniques. Each of 12 hands was mounted to a loading frame with 3 flexor tendons attached to individual pneumatic cylinders. A spring attached to a pin through the distal end of each digit provided a 1.25-kg resistance force. The force required to flex each proximal interphalangeal joint to 90 degrees was determined. Following this, the tendons were sectioned and each was repaired using a different technique so that each specimen acted as its own control. The 2- and 4-strand core sutures were placed using a suture interlock technique with radial and ulnar grasping purchase of the tendon on each side of the transverse part of the repair. Each repair was accomplished using a single core stitch with the knot buried between the tendon ends. The 4-strand repair involved an additional horizontal mattress suture with the knot buried. Repair of the dorsal side of the tendon was performed followed by core suture placement. The palmar portion of the peripheral locking suture was completed after core suture placement. Following repair, each hand was remounted on the frame and cycled 1,000 times. After cyclic loading, the resulting gap between the repaired ends of each tendon was measured, the tendons were removed from the hand, and each was loaded to failure in tension. All tendon repairs showed a small, but not statistically significant, increase in gliding resistance after reconstruction. The 2-strand repair had significantly greater gap formation after cyclic loading (mean gap, 2.75 mm) than either the 4-strand (0.30 mm) or 6-strand (0.31 mm) repair. The tensile strength of the 6-strand repair (mean, 78.7 N) was significantly greater than either the 4-strand (means, 43.0 N) or 2-strand (mean, 33.9 N) repair.

Biomechanical properties of threaded inserts for lumbar interbody spinal fusion.

Study Design Calf and human cadaveric spines were used to determine motion segment stiffness and laxity after implantation of threaded inserts (the Ray Threaded Fusion Cage, Surgical Dynamics, Inc, Concord, CA), comparing direction of placement, number of implants, shape of the device, and integrity of anterior spine structures. Stiffness and laxity of spines with inserts were compared with those with bone grafts, with and without posterior fixation plates. Objectives To determine the mechanical stabilizing properties of a threaded insert used for lumbar and lumbosacral fusion and the factors affecting stability. Summary of Background Data Limited biomechanical information has shown that implantation of these devices adds stiffness to the lumbar spine, but little information is available concerning stiffness in loading directions other than flexion and extension, the effect on stiffness of position and number of implants, and the effect of this device on motion segment laxity. Methods Mechanical properties were determined by testing lumbar vertebral motion segments in flexion, extension, lateral bending, and torsion combined with axial compressive loading. Stiffness (slope of the load/deflection curve) and neutral zone angle or laxity (angular displacement of the vertebre from no load to 1.0 Nm moment) were determined. Initial tests were performed on calf lumbar vertebrae to determine the effects of placement and number of inserts. Comparisons of bone grafts and inserts with and without supplemental plates were made using human lumbar spines. Cylindrical- and conical-shaped inserts, when placed from anterior, were tested in calf spines. The load-bearing capacity of the insert supported in calf vertebral body bone was determined. Results There was no significant effect of placement of inserts in different orientations (lateral, posterolateral, or posterior) on stiffness, excapt in torsion where posterior placement damaged facets or lamina, reducing stiffness. Placement of two inserts from posterior decreased flexion and lateral bending laxity compared with the intact motion segment. Compared with intact, bone grafts produced more stiffness only in lateral bending and had no effect on laxity. Supplemental posterior plates fixed by pedicle screws across the fusion segment increased flexion and lateral bending stiffness and reduced laxity in flexion, extension, and lateral bending. Conical-shaped inserts placed from anterior into cylindrieal holes distracted soft tissue structures, decreasing laxity. Cutting the anterior structures increased laxity by relieving some tissue tension caused by distraction. The mean maximum compressive load that could be supported by the insert was 2998 N (standard deviation = 980 N). Structural failure occurred in the supporting bone. Conclusions Threaded inserts increase vertebral motion segment stiffness and decrease laxity by distracting intervertebral structures. They are not sensitive to placement, except if vertebral structures are injured during insertion and produce constructs with more consistent mechanical properties than bone grafts.

Triangular osteosynthesis and iliosacral screw fixation for unstable sacral fractures: a cadaveric and biomechanical evaluation under cyclic loads.

Objective To conduct a biomechanical comparison of a new triangular osteosynthesis and the standard iliosacral screw osteosynthesis for unstable transforaminal sacral fractures in the immediate postoperative situation as well as in the early postoperative weight-bearing period. Design Twelve preserved human cadaveric lumbopelvic specimens were cyclicly tested in a single-limb-stance model. A transforaminal sacral fracture combined with ipsilateral superior and inferior pubic rami fractures were created and stabilized. Loads simulating muscle forces and body weight were applied. Fracture site displacement in three dimensions was evaluated using an electromagnetic motion sensor system. Intervention Specimens were randomly assigned to either an iliosacral and superior pubic ramus screw fixation or to a triangular osteosynthesis consisting of lumbopelvic stabilization (between L5 pedicle and posterior ilium) combined with iliosacral and superior pubic ramus screw fixation. Main Outcome Measures Peak loaded displacement at the fracture site was measured for assessment of initial stability. Macroscopic fracture behavior through 10,000 cycles of loading, simulating the early postoperative weight-bearing period, was classified into type 1 with minimal motion at the fracture site, type 2 with complete displacement of the inferior pubic ramus, or type 3 with catastrophic failure. Results The triangular osteosynthesis had a statistically significantly smaller displacement under initial peak loads (mean ± standard deviation [SD], 0.163 ± 0.073 cm) and therefore greater initial stability than specimens with the standard iliosacral screw fixation (mean ± SD, 0.611 ± 0.453 cm) (p = 0.0104), independent of specimen age or sex. All specimens with the triangular osteosynthesis demonstrated type 1 fracture behavior, whereas iliosacral screw fixation resulted in one type 1, two type 2, and three type 3 fracture behaviors before or at 10,000 cycles of loading. Conclusion Triangular osteosynthesis for unstable transforaminal sacral fractures provides significantly greater stability than iliosacral screw fixation under in vitro cyclic loading conditions. In vitro cyclic loading, as a limited simulation of early stages of patient mobilization in the postoperative period, allows for a time-dependent evaluation of any fracture fixation system.

Caudo-cephalad Loading of Pedicle Screws: Mechanisms of Loosening and Methods of Augmentation

The mechanism of failure and the effect of augmentation of single pedicle screws subjected to caudo-cephalad loads applied at the screw head were investigated. In each of 10 lumbar vertebrae, Steffee pedicle screws of appropriate diameter were placed in one pedicle. The cancellous bone of the contralateral pedicle was removed by curettage, a custom fabricated bushing was pressed by hand into the space, and a Steffee screw was inserted through the bushing and into the vertebral body. This arrangement was designed to transfer load directly from the pedicle screw to cortex, bypassing the cancellous bone of the pedicle. In a second experiment, a custom plate was mounted to the posterior surfaces of the articular facets allowing load transfer from the head of the screw directly to cortex. Caudo-cephalad loads were applied by a materials tester and toggle displacement (defined as the total caudo-cephalad movement of the screw under minimal load after loading through a complete cycle) was measured. Under peak loads of +199 N (caudal) and -224 N (cephalad), mean toggle at the screw head was 4.93 mm (standard deviation [SD] = 3.60 mm) for the screw alone and 4.96 mm (SD = 4.42 mm) for the screw augmented by the bushing. Screws without augmentation showed a characteristic pattern of loosening with the base of the pedicle acting as a fulcrum and a butterfly-shape void occurring in the vertebra and the pedicle where cancellous bone had been crushed. Problems with the bushing included poor fit, back-out, and split pedicles.(ABSTRACT TRUNCATED AT 250 WORDS)

Footwear style and risk of falls in older adults.

Objectives: To determine how the risk of a fall in an older adult varies in relation to style of footwear worn.

Biomechanical evaluation of methods of internal fixation of the distal humerus.

Summary: The best results following fractures of the distal humerus are provided by anatomic reduction and rigid internal fixation. Plates of two designs placed in five different fixation configurations were used to determine the construct that would maximize rigidity of fixation of the distal humerus. Using a cadaver distal humerus osteotomy, with and then without cortical contact, motion of the distal fragment was measured with respect to the proximal fragment in axial and torsional loading, anterior to posterior and posterior to anterior bending, and lateral to medial and medial to lateral bending. With cortical contact, two plates when placed medial and lateral or at 90° to each other provided equivalent rigidity. However, with a cortical gap, the combination of a specially designed anatomic lateral buttress “J” plate and a medial reconstruction plate gave the greatest rigidity (ANOVA, p < 0.05). Two-plate constructs do not require placement at 90° to obtain sufficient rigidity, but do require placement on separate bony pillars and different surfaces.

Nerve tension and blood flow in a rat model of immediate and delayed repairs

In vivo studies of rat sciatic nerves in models of immediate and delayed repairs demonstrated the viscoelastic properties of the nerve and the inverse correlation between nerve blood flow and tension. In both the proximal and distal segments of the divided nerve in models of immediate and delayed repairs, the nerve blood flow decreased approximately 50% with substantial recovery in 30 minutes after 8% elongation, whereas 15% elongation produced approximately an 80% reduction in blood flow with minimal recovery. However, the baseline blood flow of the nerves in the delayed-repair model was nearly two times higher than that of the acutely injured nerves. Maximal decrease in nerve tension and corresponding increase in blood flow occurred within the first 20 minutes after elongation. The suture pull-out with failure of the repairs occurred at more than 15% elongation for all nerves. The previously divided nerves had a sixfold greater decrease in length than the acutely divided nerves (p less than 0.02). For repairs of large nerves where vascular ingrowth is likely to be incomplete, elongation of more than 8% may cause ischemia that is detrimental to nerve regeneration. Mechanical failure of the repairs occurs after elongation of 16% to 17%. The combination of nerve ischemia and mechanical failure of suture repairs suggests that surgeons should be careful to limit the use of elongation in acute and delayed repairs.

The effects of distal radius fracture malalignment on forearm rotation: A cadaveric study

Seven fresh cadaveric specimens were used to determine the loss of forearm rotation with varying distal radius fracture malalignment patterns. Uniplanar malunion patterns consisting of dorsal tilt, radioulnar translation, or radial shortening were simulated by creating an osteotomy at the distal end of the radius, orienting the distal fragment position using an external fixator, and maintaining the position with wedges and a T-plate. Rotation of the forearm was produced by fixing the elbow in a flexed position and applying a constant torque to the forearm using deadweights. Forearm rotation was measured with a protractor. Dorsal tilt to 30 degrees and radial translation to 10 mm led to no significant restriction in forearm pronation or supination ranges of motion. A 5-mm ulnar translation deformity resulted in a mean 23% loss of pronation range of motion. Radial shortening of 10 mm reduced forearm pronation by 47% and supination by 29%.