Randal P. Ching

The Effect of Posterior Tibial Tendon Dysfunction on Hindfoot Kinematics

This biomechanical study investigated the functional role of the posterior tibial tendon (PTT) in acquired flatfoot mechanics. Acquired flatfoot deformity has been attributed to PTT dysfunction; however, the progression from acute dysfunction to end-stage deformity has not been fully demonstrated. Eight human cadaver lower leg and foot specimens were used in two phases of experimental testing. In Phase 1, intact (normal) specimens were loaded to simulate (a) heel strike, (b) stance, and (c) heel rise both with and without PTT function. Then, each specimen was subjected to a procedure designed to create a simulated flatfoot deformity. The resulting flattened feet were used in Phase 2 to examine the effect of restoring PTT function to a flatfoot model. During both phases of testing, the 3-D kinematic orientation of the hindfoot complex was recorded. Small but statistically significant changes in the angular orientation of the hindfoot complex were observed, during both Phase 1 and 2 testing, when comparing the effects of a functional and dysfunctional PTT. The greatest angular changes were recorded during heel rise. For the normal foot, the small changes observed in the orientation of the hindfoot complex following release of the PTT load suggest that the intact osteo-ligamentous structure of the hindfoot is initially able to maintain normal alignment following acute PTT dysfunction. Once the soft tissues have been weakened, as in our flatfoot model, the PTT had little effect in overcoming the soft tissue laxity to correct the position of the foot.

Mechanism of the burst fracture in the thoracolumbar spine. The effect of loading rate.

Study Design Calf lumbar spine motion segments were randomly assigned to two groups. After insertion of a transducer capable of measururing transient occlusion of the spinal canal during impact, a low rate axial impact was applied in one group and a high rate load in the other.Post -injury computed tomography to determine the effect of rate of load application on occlusion of the spinal canal. Objectives This study was designed to determine if for the same direction of impact and total energy delivered, occlusion of the spinal canal postvertebral fracture was related to the rate at which the impact was delivered (time from zero to peak load). Summary of Background Data Several reports based on clinical observation have hypothesized that axial burstfractures, which displace bone fragments into the canal. occur because of internal pressurization and explosion of the vertebral body.The extent of bursting of the body, which could be related to the rate at which the load is applied. Method Using calf lumbar spines, a transducer was placed within the spinal canal, after removal of the cord, to measure canal occlusion during impact. One group received axial compressive impacts at a mean loading rate of 400 msec (zero to peak load) using a materials-tesing machine. The energy of failure was determined and used to select a drop weight and distance for the high loading rate tests, which would yield equivalent impact energy. The second group received impacts at a loading rate of 20 msec.The pos-injury radiographs and canal occlusion measurements were compared. Results The same mean energy of impact was used in the fractures for both groups. Post-injury radiographs of the low loading rate group showed compressive fractures with a mean canal occlusion of 6.84%, whereas the high loading rate group had burst fractures with mean canal encroachment of 47.6% (P=0.0007> Conclusions For the same energy and direction of impact, a high impact loading rate produces fractures with significant canal encroachment, whereas minimal encoachment is seen for fractures produced at a low loading rate.

Neural space integrity of the lower cervical spine: effect of normal range of motion.

Study Design. An experimental investigation of intervertebral foramen and spinal canal neural space integrity was performed throughout physiologic range of motion of the lower cervical spine in intact human cadaver specimens. Objective. To investigate cervical positions that might place the neural tissues of the spine in heightened risk of injury. To meet this objective the following hypotheses were tested: 1) spinal canal integrity varies with specific normal range of motion positions of the lower cervical spine, and 2) intervertebral foramen integrity is dependent on and unique for different physiologic positions of the lower cervical spine. Summary of Background Data. Cervical spine injuries are frequently associated with compressive damage to neurologic tissues and consequently poor clinical outcomes. Neurologic injury typically occurs from disc, ligamentous, or bony occlusion of the spinal canal and intervertebral foraminal spaces dynamically during an injury event or with abnormal alignment and position after the injury event. Prior studies have shown pressure and geometric changes in cervical spine neural spaces in certain cervical spine positions. However, to the authors’ knowledge, this is the first research effort aimed at elucidating the integrity of the cervical spine neural spaces throughout the normal physiologic range of motion. Methods. The authors instrumented 17 fresh-frozen unembalmed cadaveric human cervical spines (C3–C7) with specially designed intervertebral foramen occlusion transducers and a spinal canal occlusion transducer. The specimens were loaded with pure bending moments to produce simulated physiologic motions of the lower cervical spine. The resulting occlusion profiles for the intervertebral foramen and spinal canal were recorded along with the 6-degree of freedom position of the cervical spine. Because these occlusion measurements describe the ability of the spine to preserve the space for the neural structures, the authors define this neuroprotective role of the vertebral column as neural space integrity. Results. The range of motion developed experimentally in this study compared well with published reports of normal cervical motion. Thus, subsequent changes in neural space integrity may be regarded as resulting from normal human cervical spine motion. No significant change in the spinal canal space was detected for any physiologic motion; however, intervertebral foramen integrity was significantly altered in extension, ipsilateral bending, combined ipsilateral bending and extension, and combined contralateral bending with extension when compared with intact upright neutral position. Conclusions. This study defines the range of neural space integrity associated with simulated physiologic motion of the lower cervical spine in an experimental setting. This information may be useful in comparing neural space changes in pathologic conditions and may enhance refinement of neurologic injury prevention strategies.

Geometric changes in the cervical spinal canal during impact.

Summary of Background Data Although the extant of injury after cervical spine fracture can be visualized by imaging, the deformations that occur in the spinal canal during injury are unknown. Study Design This study compared spinal canal occlusion and axial length changes occurring during a simulated compressive burst frecture with the residual deformations after the injury. Methods Canal occlusion was measured from changes in pressure in a flexible tube with fluid flowing through it, placed in the canal space after removal of the cord in cadaver specimens. To measure canal axial length, cables were fixed in C1 and led through the foramen transversarium from C2-T1, then out through the base, where they were connected to the core rods of linearly variable differential transformers (LVDT). Axial compressive burst fractures were created in each of ten cadaveric cervical spine specimens using a drop-weight, while force, distraction, and occlusion were monitored throughout theinjury event. Pre- and postinjury radiographs and computed tomography scans compared transient and post-injury spinal canal geometry changes. Results In all cases, severe compressive injuries were produced. Three had an extension component in addition to compression of the vertabra and retropulsion of bone into the canal. The mean post-injury axial height loss measured from radiographs was only 35% of that measured transiently (3.1 mm post-injury, compared with 8.9 mm measured transiently), indicating significant recovery of axial height after impact. Post-injury and transient height loss were not significantly correlated (r2 = 0.230, P = 0.16) demonstrating that it is not a good measure of the extent of injury. Similarly, mean post injry canal area was 139% of the minimum area measured during impact indicating recovery of canal space, and post-injury and transient values were not significantly correlated (r2 = 0.272, P = 0.12). Mean post-injury midsagittal diameter was 269% of the minimum transient diameter and showed a weak but significant correlation (r2 = 0.481, P = 0.03). Conclusions Two potential spinal cord injury-causing mechanisms in axial bursting injuries of the cervical splne are occlusion and shortening of the canal. Post-injury radiographic measurements significantly underestimate the actual transient injury that occurs during impact.

Effect of loading rate on endplate and vertebral body strength in human lumbar vertebrae

Previous studies have implied that increases in loading rate resulted in changes in vertebral mechanical properties and these changes were causative factors in the different fracture types seen with high-speed events. Thus many researchers have explored the vertebral body response under various loading rate conditions. No other study has investigated the role of the endplate in high-speed vertebral injuries. The current study determined changes in the endplate and vertebral body strength with increases in displacement rate. The endplate and vertebral body failure loads in individual lumbar vertebrae were documented for two displacement rates: 10 and 2500 mm/s. Using cross-sectional areas from the endplate and vertebral body, failure stresses for both components were calculated and compared. Both the endplate and vertebral body failure loads increased significantly with increased loading rate (p<0.005). Although the vertebral body failure stress increased significantly with loading rate as well (p<0.01), the endplate stresses did not (p>0.35). In addition, the endplate and vertebral strengths were not significantly different under high-speed loading (p>0.60), which inhibits possible predictions as to which bony component would fail initially during a high-speed injury event. It is possible that load distribution may contribute more to the fracture patterns seen at high speeds over vertebral component strength.

DAMAGE TO BICYCLE HELMETS INVOLVED WITH CRASHES

The objective was to evaluate the relationship between helmet damage and head injuries in helmeted bicyclists in a sub-study of a large case-control study of bicycle injuries and helmet effectiveness. The setting consisted of seven hospital emergency departments in Seattle, WA. Hospitalized patients and medical examiners cases were included. The participants in the study were helmeted bicyclists who suffered a head injury or who damaged or hit their helmet in a crash. The Snell Memorial Foundation laboratory evaluated the helmets, blinded to crash circumstance and injury diagnosis. Damage was scored on a five-point scale (0 = none to 4 = destroyed). The damage location for each helmet was coded into regions (six longitudinal and three latitudinal) and mapped onto a three-dimensional CAD (computer-aided design) model of a helmet. The same procedure was also followed for injury location, which was mapped onto a three-dimensional ISO (International Organization for Standardization) headform for visualization of head-injury distribution. 785 helmeted subjects met the criteria for inclusion in the sub-study, and 527 helmets were purchased and evaluated (67%). 316 (60%) of the helmets had no or minimal damage, and 209 (39.7%) had significant damage (score 2, 3 or 4). Helmet types were 49.7% hard shell, 34.2% thin shell and 16.1% no shell. The risk of head and brain injury increased if the helmet was destroyed: OR = 5.3 (95% CI 2.9, 9.9) and OR = 11.2 (95% CI 3.5, 37.9), respectively. A high proportion of helmet impacts were along the front edge of the helmet, with a preponderance of head injuries in the same region. The large number of impacts to the front rim of the helmet, combined with the substantial number of riders with injuries to the forehead, indicate that some helmets, because of poor fit or wearing style, expose the forehead to injury. In addition, the data indicate that for a small proportion of injuries, the energy to the helmet may exceed design limits.

Canal geometry changes associated with axial compressive cervical spine fracture

Study Design. A laboratory study using isolated ligamentous human cadaveric cervical spines to investigate canal occlusion during (transient) and after (steady-state) axial compressive fracture. Objectives. To determine whether differences exist between transient and postinjury canal occlusion under axial compressive loading, and to examine the effect of loading rate on canal occlusion. Summary of Background Data. Prior studies have shown no correlation between neurologic deficit and canal occlusion measurements made on radiographs and computed tomography scans. The authors hypothesized that postinjury radiographic assessment does not provide an appreciation for the transient occlusion that occurs during the traumatic fracture event, which may significantly affect the neurologic outcome. Methods. Twelve human cervical spines were instrumented with a specially designed canal occlusion transducer, which dynamically monitored canal occlusion during axial compressive impact. Six specimens were subjected to a fast-loading rate (time to peak load, ∼20 msec), and the other six were subjected to a slow-loading rate (time to peak load, ∼250 msec). After impact, two different postinjury canal occlusion measurements were performed. Results. Each of the six specimens subjected to the fast-loading rate incurred burst fractures, whereas the slow-loading rate produced six wedge-compression fractures. For the fast-rate group, the postinjury occlusionmeasurements were significantly smaller than the transient occlusion. In contrast, transient occlusion was not found to be significantly different from postinjury occlusion in the slow-rate group. All of the comparisons between loading rate groups showed significant differences, with the fast-rate fractures producing larger amounts of canal occlusion in every category. Conclusions. The findings indicate that even if canal occlusion could be measured immediately after axial compressive trauma, the measurement would underestimate the maximal amount of transient canal occlusion. Therefore, postinjury measurement of canal occlusion may indicate a smaller degree of neurologic deficit than what might be expected if the transient occlusion could be measured.

Biomechanics of procedures used in adult flatfoot deformity

A flatfoot deformity can occur secondary to fairly obvious causes, or more subtle and less definable entities. Complicating the situation further, it is likely that the cause of an acquired flatfoot deformity in an adult is multifactorial. This likelihood makes the definition, diagnosis, and appropriate treatment of this condition a daunting task. More research is needed to define further the biomechanics of the foot and to understand the significance of the forces that combine to create flatfoot deformity.

Biomechanical analysis of the first metatarsocuneiform arthrodesis

This investigation was designed to help define the unique loading characteristics of the first metatarsocuneiform arthrodesis procedure. Part I of this investigation employed nine fresh frozen, matched-pair cadaveric specimens. One specimen in each pair had the subchondral plate removed from the opposing joint surfaces, while the remaining specimen had only the articular cartilage removed. All specimens were stabilized in an identical manner utilizing two 3.5-mm cortical screws. Part II of the investigation also utilized nine fresh frozen, matched-pair cadaveric specimens. Only the articular cartilage was removed prior to placement of fixation. All specimens were stabilized with two crossing 3.5-mm cortical screws. Placement of a third screw was randomized between specimens of a matched pair. Specimens were loaded to failure in cantilever bending utilizing a materials tester. There was a statistically significant (p = .04) greater load to failure and bending moment in specimens with an intact subchondral plate. Values for construct stiffness were not found to be significantly different (p = .95) between specimens with and without a subchondral plate. Although the addition of a third screw increased the load to failure and bending moments, differences were not found to be statistically different (p = .11-.21) from two screws. Preserving the subchondral plate will enhance the stability of the first metatarsocuneiform arthrodesis. Two or three screws can be employed to shield the fusion site from loading; however, three screws were shown to be more effective than two.

Muscular imbalances resulting in a clawed hallux.

A clawed hallux is a deformity of the great toe resulting from a muscular imbalance. Using a cadaveric foot-loading frame, we quantitatively assessed the role of the peroneus longus (PL), extensor hallucis longus (EHL), and flexor hallucis longus (FHL) on position and pressure distribution of the first ray by simulating muscle imbalances. The experimental protocol included applying seven different combinations of simulated disproportionate loads (“overpulls”) for these three muscles using midstance force values derived from the literature. This study quantified the angular change in the joints of the first ray and measured the plantar pressure beneath the head of the first metatarsal and the hallux. The results indicated that the peroneus longus was statistically the greatest contributor to the elevation of plantar pressure beneath the first metatarsal, while the EHL and FHL were primarily responsible for the angular changes resulting in the clawed hallux deformity.