Tait S. Smith

Short-segment pedicle instrumentation. Biomechanical analysis of supplemental hook fixation

Study Design This biomechanical study of fractures in cadaver vertebrae used specially designed pedicle screws to determine screw strains during loading of two different fixation constructs. Objectives The authors determined the relative benefit of adding offset sublaminar hooks to standard pedicle screw constructs to reduce screw bending moments and prevent fixation failure and sagittal collapse. Summary of Background Data Clinical studies have demonstrated a high incidence of early screw failure in short-segment pedicle instrumentation constructs used to treat unstable burst fractures. Strategies to prevent early construct failure include longer constructs, anterior strut graft reconstruction, and use of offset sublaminar hooks at the ends of standard short-segment pedicle instrumentation constructs. Methods Human cadaver spines with an L1 burst fracture were instrumented with a standard short-segment pedicle instrumentation construct using specially instrumented pedicle screws. Mechanical testing was carried out in flexion, extension, side bending, and torsion, and stiffness and screw bending moments were recorded. Offset hooks were applied initially, then removed and testing repeated. Stiffness data were compared to intact and postfracture results, and between augmented and standard constructs. Results Addition of offset laminar hooks, supralaminar at T11 and infralaminar at L2, to standard short-segment pedicle instrumentation constructs increased stiffness in flexion by 268%, in extension by 223%, in side bending by 161%, and in torsion by 155% (all were significant except torsion). Sublaminar hooks also reduced pedicle screw bending moments to roughly 50% of standard in both flexion and extension (P < 0.05). Conclusions Supplemental offset hooks significantly increase construct stiffness without sacrificing principles of short-segment pedicle instrumentation, and absorb some part of the construct strain, thereby reducing pedicle screw bending moments and the likelihood of postyield deformation and clinical failure.

Pathomechanics of Closed Rupture of the Flexor Tendon Pulleys in Rock Climbers

We performed a study on twenty-one cadaveric fingers (seven non-paired forearms) to determine the pathomechanics of closed traumatic rupture of the flexor tendon pulleys in rock climbers. The ages of the individuals at the time of death ranged from sixty-one to eighty-four years (mean, seventy-four years). The forearm was placed in a custom-made loading apparatus, and individual fingers were tested separately under simulated in vivo loading conditions. The flexor digitorum superficialis and profundus tendons of each digit were attached to computer-controlled linear stepper motors that were equipped with force transducers, and the force in the tendons was simultaneously increased until avulsion of the tendons or osseous failure occurred. The force in the tendons, the excursion of the tendons, and the force at the fingertip were measured. Damage to the pulleys and bowstringing of the tendons were visualized with a fiberoptic camera. Two fingers fractured before complete rupture of the pulleys. Seventeen of the remaining nineteen fingers sustained an isolated rupture of either the A2 or the A4 pulley as the initial failure event; the A4 pulley ruptured first in fourteen digits (p < 0.001). The A3 and A4 pulleys ruptured simultaneously in one finger, and the A2, A3, and A4 pulleys ruptured simultaneously in another. Subtle bowstringing of the flexor digitorum profundus tendon occurred only after two consecutive pulleys had ruptured (either the A2 and A3 pulleys or the A3 and A4 pulleys). Rupture of all three pulleys was required to produce obvious bowstringing. Isolated rupture of the A2 or A4 pulley did not result in detectable bowstringing of the flexor digitorum profundus tendon. The A1 pulley always remained intact. CLINICAL RELEVANCE: Bowstringing of the flexor digitorum profundus tendon across the proximal interphalangeal joint with resisted flexion of the fingertips has been considered diagnostic for isolated closed rupture of the A2 pulley. The results of the present study, however, suggest that isolated injury of the A2 pulley rarely occurs. On the basis of our findings, we believe that reliance on bowstringing of the tendon at the proximal interphalangeal joint as an indicator of an isolated rupture of the A2 or A4 pulley may be misleading.

Strain and loading of the second metatarsal during heel-lift.

Metatarsal stress fractures occur in military recruits after long marches and in athletes after episodes of overtraining involving running or jumping. It has been demonstrated that contraction of the plantar flexors of the toes helps to counteract the moments placed on the metatarsals by body weight. It is possible that physiological fatigue due to strenuous or repetitive exercise reduces the rate and force of contraction of the plantar flexors, thereby increasing metatarsal strain per cycle, and that this mechanism is the primary cause of stress fractures of these bones. To test the hypothesis that fatigue of the plantar flexors causes increased metatarsal loading, thereby predisposing these bones to stress fracture, we measured metatarsal strains in nine fresh cadaveric feet with use of an apparatus that simulated physiological loading due to body weight as well as contraction of the plantar flexors. Each foot was loaded to 750 newtons of ground-reaction force by simulated contraction of the triceps surae, and strains were recorded in the mid-part of the shaft of the second metatarsal. Tests were repeated with use of simulated activity of different combinations of the flexor digitorum longus, flexor hallucis longus, peroneus brevis, peroneus longus, and tibialis posterior muscles. In situ bending moments and axial loads subsequently were derived for each configuration. Dorsal strain was significantly reduced by simulated contraction of the flexor hallucis longus. Plantar-dorsal bending was significantly reduced by simulated contraction of the flexor digitorum longus.

Influence of Extrinsic Plantar Flexors on Forefoot Loading During Heel Rise

This investigation studied the effects of simulated plantar flexor muscle activity on forefoot loading using a static cadaver model. Nine cadaver feet were mounted in an apparatus in the heel rise position. Using computer-controlled and pneumatic actuators, forces were simultaneously applied to the tendons of the triceps surae, flexor hallucis longus, flexor digitorum longus, peroneus brevis and longus, and tibialis posterior until 750 N of ground reaction force was achieved, at which time forefoot plantar pressure patterns were captured immediately with a pedobarograph. Second metatarsal bending moments were calculated from strain gauge data collected concurrently. Consecutive loading cycles were performed with sequential elimination of simulated muscle force from each tendon except the Achilles. Loss of simulated flexor hallucis longus activity significantly decreased great toe contact forces and significantly increased forces under the forefoot. Simulated loss of both the flexor hallucis longus and flexor digitorum longus caused significant decreases in contact area, pressure, and force beneath the toes and significant increases in contact area and force under the forefoot. Bending moments in the second metatarsal were shown to vary directly with peak pressure under the second metatarsal head (r = 0.801). These findings demonstrate the load distributing function of the extrinsic plantar flexors during heel rise.

S1 screw bending moment with posterior spinal instrumentation across the lumbosacral junction after unilateral iliac crest harvest

Study Design. A biomechanical study comparing fixation across the lumbosacral junction. Objectives. To determine which long posterior construct across the lumbosacral junction produces the least bending moment on the S1 screw when only one ilium is available for fixation. Summary of Background Data. Recent in vitro studies have demonstrated the benefit of anterior support and fixation into the ilium when instrumenting a long posterior construct across the lumbosacral junction. Methods. Four L2–sacrum constructs were tested on six synthetic models of the lumbar spine and pelvis simulating that the right ilium had been harvested. Construct 1: L2–S1 bilateral screws. Construct 2: L2–S1 + left iliac bolt. Construct 3: L2–S1 + left iliac bolt + right S2 screw. Construct 4: L2–S1 + bilateral S2 screws. The four constructs were then retested with an anterior L5–S1 strut. A flexion–extension moment was applied across each construct, and the moment at the left and right S1 pedicle screw was measured with internal strain gauges. Results. Iliac bolt fixation was found to significantly decrease the flexion–extension moment on the ipsilateral S1 screw by 70% and the contralateral screw by 26%. An anterior L5–S1 strut significantly decreased the S1 screw flexion–extension moment by 33%. Anterior support at L5–S1 provided no statistical decrease in the flexion–extension moment when bilateral posterior fixation beyond S1 was present with either a unilateral iliac bolt and contralateral S2 screw, or bilateral S2 screws. Conclusions. There is a significant decrease in the flexion–extension moment on the S1 screw when extending long posterior constructs to either the ilium or S2 sacral screw. There is no biomechanical advantage of the iliac bolt over the S2 screw in decreasing the moment on the S1 screw in flexion and extension. Adding anterior support to long posterior constructs significantly decreases the moment on the S1 screw. Adding distal posterior fixation to either the ilium or S2 decreases the moment on S1 screws more than adding anterior support. Further, adding anterior support when bilateral distal fixation past S1 is already present does not significantly decrease the moment on the S1 screws in flexion and extension.

Optimal slicing of free-form surfaces

Many applications, such as contour machining, rapid prototyping, and reverse engineering by laser scanner or coordinate measuring machine, involve sampling of free-from surfaces along section cuts by a family of parallel planes with equidistant spacing ∆ and common normal N. To ensure that such planar sections provide faithful descriptions of the shape of a surface, it is desirable to choose the relative orientation that maximizes, over the entire surface, the minimum angle between N and the local surface normal n. We address this optimization problem by computing the (symmetrized) Gauss map for the surface, projecting it stereographically onto a plane, and invoking the medial axis transform for the complement of its image to identify the orientation N that is “most distant” from the symmetrized Gauss map boundary. Using a Gauss map algorithm described elsewhere, the method is implemented in the context of bicubic Bezier surfaces, and applied to the problem of minimizing the greatest scallop height incurred in contour machining of surfaces using a 3-axis milling machine with a ball-end cutter.  2001 Elsevier Science B.V. All rights reserved.

Patellar strain and patellofemoral contact after bone-patellar tendon-bone harvest for anterior cruciate ligament reconstruction

OBJECTIVE To characterize the morbific consequences of harvesting a patellar tendon graft for use in reconstructing the anterior cruciate ligament (ACL) of the knee, specifically, (1) to measure changes in patellar strain and patellofemoral contact due to graft harvest, (2) to evaluate the ability of bone-grafting the patellar defect to mitigate these effects, and (3) to characterize failure of the extensor mechanism after harvest of a patellar tendon graft. DESIGN Twenty-two cadaver knee joints were tested before and after harvest of a patellar tendon graft and after filling the patellar defect with polymethylmethacrylate to simulate a healed bone graft, Knees were positioned in 30 degrees, 60 degrees, and 90 degrees flexion and loaded while measuring axial strain in the anterior patella and patellofemoral contact. Knees were then loaded to failure. RESULTS Harvest of the graft produced increases in axial strain at all flexion angles. Filling the defect restored axial strain to normal values. Patellofemoral contact in the presence of a defect, either filled or empty, was not different from contact for intact patellae. Most knees failed by transpatellar fracture; mean extension moment at failure was 112.8Nm. The best predictors of failure were age and gender. CONCLUSION Patients undergoing ACL reconstruction with a patellar tendon graft are at increased risk of anterior knee pain and disruption of the extensor mechanism. Bone-grafting the patellar defect created by graft harvest can reduce these risks. Our findings underscore the importance of carefully controlled rehabilitation and suggest that if an accelerated program of rehabilitation is anticipated, the patellar defect should be bone-grafted. Older patients, particularly women, are at increased risk of catastrophic failure of the knee extensor mechanism after ACL reconstruction using patellar tendon graft.

Gauss map computation for free-form surfaces

The Gauss map of a smooth doubly-curved surface characterizes the range of variation of the surface normal as an area on the unit sphere. An algorithm to approximate the Gauss map boundary to any desired accuracy is presented, in the context of a tensor-product polynomial surface patch, r(u,v) for (u,v)@?[0,1]x[0,1]. Boundary segments of the Gauss map correspond to variations of the normal along the patch boundary or the parabolic lines (loci of vanishing Gaussian curvature) on the surface. To compute the latter, points of vanishing Gaussian curvature are identified with the zero-set of a bivariate polynomial, expressed in the numerically-stable Bernstein basis-the subdivision and variation-diminishing properties then govern an adaptive quadtree decomposition of the (u,v) parameter domain that captures the zero-set of this polynomial to any desired accuracy. Loci on the unit sphere corresponding to the patch boundaries and parabolic lines are trimmed at their mutual or self-intersection points (if any), and the resulting segments are arranged in a graph structure with the segment end-points as nodes. By appropriate traversal of this graph, the Gauss map boundary segments may then be identified in proper order, and extraneous segments (lying in the Gauss map interior) are discarded. The symmetrization of the Gauss map (by identification of antipodal points) and its stereographic projection onto a plane are also discussed.

A device for the measurement of pedicle screw moments by means of internal strain gauges

Pedicle screws are commonly used in spinal reconstruction, and failure of pedicle screws due to bending is a significant clinical problem. To measure the moments typically placed on pedicle screws in situ we instrumented 7 mm Cotrel-Dubousset (CD) pedicle screws with internally mounted strain gauges. The screws were designed to measure flexion-extension moments at a single cross-section as dictated by strain gauge placement. It is possible to measure moments of up to 12 Nm at any location along the length of the screw by constructing transducers with varying strain gauge placements. These transducers are capable of measuring moments at points located within the vertebra including the pedicle, which is where failure usually occurs clinically. Transducer output was both linear and reproducible. These transducers are being used to investigate the load transfer characteristics between the pedicle screw and the vertebra. This technique could be applied to investigations of load sharing in reconstruction plates, lag-screws, and cross-locked intramedullary nails.

Modified transverse locking nail fixation of proximal femoral fractures.

It was hypothesized that transverse locking screws of intramedullary nails, seated above the lesser trochanter, provide equal strength to that of reconstruction nails, and that screws placed through the medial cortex of the femoral neck do not have adverse biomechanical effects during physiologic loading. Synthetic femurs (n = 10) and paired anatomic specimen femurs (n = 14) were tested intact and with an intramedullary device in place. Intact specimens were loaded nondestructively, then a segmental subtrochanteric defect was created and either a high seated transverse locking nail or a reconstruction nail was inserted and statistically locked. Axial and torsional stiffness were determined followed by axial failure testing. Mechanical parameters evaluated were stiffness, displacement, and energy. The implanted specimens did not show any statistically significant difference between transverse or reconstruction screw constructs with any of the measured parameters (stiffness, displacement, and energy). Failure tests in implanted specimens also did not show any statistically significant difference in yield load, yield displacement, or energy to failure between implant constructs. All anatomic specimens failed, with fractures of the proximal fragment involving medial and lateral cortices. Synthetic specimens did not fracture but showed failure with implant deformation at the level of the skeletal defect. The use of high seated transverse locking nails for complex proximal femoral fractures is a viable option and has comparable in vitro mechanical performance with reconstruction nails. Although not shown to be a problem in the present study, clinical evaluation of screws through the medial femoral neck cortex is required.