A Direct Method for Mapping the Center of Pressure Measured by an Insole Pressure Sensor System to the Shoe's Local Coordinate System.

A direct method to express the center of pressure (CoP) measured by an insole pressure sensor system (IPSS) into a known coordinate system measured by motion tracking equipment is presented. A custom probe was constructed with reflective markers to allow its tip to be precisely tracked with motion tracking equipment. This probe was utilized to activate individual sensors on an IPSS that was placed in a shoe fitted with reflective markers used to establish a local shoe coordinate system. When pressed onto the IPSS the location of the probe's tip was coincident with the CoP measured by the IPSS (IPSS-CoP). Two separate pushes (i.e., data points) were used to develop vectors in each respective coordinate system. Simple vector mathematics determined the rotational and translational components of the transformation matrix needed to express the IPSS-CoP into the local shoe coordinate system. Validation was performed by comparing IPSS-CoP with an embedded force plate measured CoP (FP-CoP) from data gathered during kinematic trials. Six male subjects stood on an embedded FP and performed anterior/posterior (AP) sway, internal rotation, and external rotation of the body relative to a firmly planted foot. The IPSS-CoP was highly correlated with the FP-CoP for all motions, root mean square errors (RMSRRs) were comparable to other research, and there were no statistical differences between the displacement of the IPSS-CoP and FP-CoP for both the AP and medial/lateral (ML) axes, respectively. The results demonstrated that this methodology could be utilized to determine the transformation variables need to express IPSS-CoP into a known coordinate system measured by motion tracking equipment and that these variables can be determined outside the laboratory anywhere motion tracking equipment is available.

A Biomechanical Assessment of Shaken Baby Syndrome: What About the Spine?

BACKGROUND: Shaken baby syndrome occurs following inertial loading of the pediatric head, resulting in retinal hemorrhaging, subdural hematoma, and encephalopathy. However, the anatomically vulnerable cervical spine receives little attention. Automotive safety literature is replete with biomechanical data involving forward-facing pediatric surrogates in frontal collisions, an environment analogous to shaking. Publicly available data involving child occupants were utilized to study pediatric neck and head injury potential. We hypothesized that inertial loading provides a greater risk of injury to the cervical spine than to the head. METHODS: Full-scale automotive crash tests (n = 131) and deceleration sled tests (n = 32) utilizing forward-facing 3-year-old surrogates with head accelerometers and cervical force sensors were analyzed. One hundred sixty-seven full-scale vehicle and 33 sled test runs were assessed in the context of published injury assessment reference values (IARVs) for closed head injury (head injury criterion 15 [HIC15]) and cervical tensile strength in the 3-year-old model. RESULTS: One hundred sixty-one (96%) child surrogates in full-scale crash tests exceeded the cervical peak tension IARV, while only 37 (22%) surpassed the HIC15 IARV. Similarly, in sled testing runs, 27 (82%) pediatric surrogates exceeded cervical tension IARVs, while 1 (3%) surpassed the HIC15 IARV. In both full-scale and sled tests, all surrogates surpassing the HIC15 IARV also exceeded the cervical tension IARV. Positive linear correlations were observed between HIC15 and cervical tensile forces in both full-scale vehicle (R2 = 0.15) and sled testing runs (R2 = 0.54). CONCLUSIONS: These data support the hypothesis that inertial loading of the head provides a greater injury risk to the cervical spine than to closed-head injury.

Enforced exercise after blunt trauma significantly affects biomechanical and histological changes in rabbit retro-patellar cartilage.

Our laboratory has developed an animal model to study factors leading to chronic disease in a blunt impacted joint. Studies to date indicate post trauma softening of the impacted joint cartilage, but a limited degree of histological degradation in the tissue. The model utilizes treadmill exercise of the animal post trauma. The hypothesis of the current study was that post trauma exercise helps limit histological and mechanical degradation of the impacted retro-patellar cartilage. The study involved a group of animals with enforced exercise on a treadmill and another group with cage-activity post trauma. The animals were sacrificed after 24 months. Mechanical and histological analyses were performed on the retro-patellar cartilage from each group. The impacted versus contra-lateral, non-impacted retro-patellar cartilage was mechanically softened in the exercise group, but not in the cage-activity group. Histological analyses of the tissue from the cage-activity group indicated that this cartilage had less surface integrity, more ossification/calcification, and more erosion than that in the impacted tissue from the exercise group. These tissue changes may lead to an apparent stiffening effect in the impacted cartilage from the cage-activity group at 24 months post-trauma. Potential relationships between the intensity and frequency of post trauma exercise and the mechanical character and histological degradation of the impacted cartilage need additional study. The study indicates that post-trauma exercise can significantly alter the outcome of a blunt knee joint trauma in this experimental animal model.

Chronic changes in rabbit retro-patellar cartilage and subchondral bone after blunt impact loading of the patellofemoral joint.

Animal models of acute joint injury are useful for study of changes in joint tissues that may eventually lead to degradative disease. Our laboratory has developed a joint trauma model using a single blunt impact to the patellofemoral joint of rabbits and documented softening of retro-patellar cartilage and thickening of its underlying bone out to 12 months post-trauma. In the present study, we examined changes in these joint tissues out to 36 months post-impact. Forty-nine Flemish giant rabbits were impacted on the right patellofemoral joint and sacrificed at one of six times: 0, 4.5, 7.5, 12, 24, and 36 months post-impact. A 30% reduction in the compressive modulus of traumatized retro-patellar cartilage occurred at 4.5 months versus the contralateral, non-impacted limb and remained at this reduced level out to 36 months. The fluid permeability of traumatized cartilage also increased over time from baseline and versus the non-impacted limb. Tissue thickness increased slightly at 4.5 months and then decreased over time to a 45% difference from baseline at 36 months post-trauma. While impacted cartilage revealed a significantly greater length of surface fissuring than contralateral, non-impacted cartilage, no time-dependent changes were evident in this study. Moreover, the number and depth of these impact surface lesions did not change as a function of time. Finally, the histological analyses indicated that the thickness of underlying subchondral bone increased over time from baseline and versus that in the non-impacted limb. This long-term study suggested an association between a decrease in the characteristic time constant of traumatized cartilage and thickening of the underlying subchondral bone. Any potential cause and effect relationship, however, must be investigated in future studies.

Impact orientation can significantly affect the outcome of a blunt impact to the rabbit patellofemoral joint.

This laboratory has developed a subfracture, joint trauma model in rabbits. Using a dropped impact mass directed onto a slightly abducted joint, chronic softening of retropatellar cartilage and thickening of underlying subchondral bone are documented in studies to 1 year post-insult. It has been hypothesized that these tissue changes are initiated by stresses developed during impact loading. A previous analytical study by this laboratory suggests that tensile strains in retropatellar cartilage can be significantly lowered, without significantly changing the intensity of stresses in the underlying subchondral bone, by reorientation of patellar impact more centrally on the joint. In the current study comparative experiments were performed on groups of animals after either an impact directed on the slightly abducted limb or a more central impact. One-year post-trauma in animals subjected to the central-oriented impact no degradation of the shear modulus for the retropatellar cartilage was documented, but the thickness of the underlying subchondral bone was significantly increased. In contrast, alterations in cartilage and underlying bone following impact on the slightly abducted limb were consistent with previous studies. The current experimental investigation showed the sensitivity of post-trauma alterations in joint tissues to slight changes in the orientation of impact load on the joint. Interestingly, for this trauma model thickening of the underlying subchondral plate occurred without mechanical degradation of the overlying articular cartilage. This supports the current laboratory hypothesis that alterations in the subchondral bone and overlying cartilage occur independently in this animal model.

Determination of dynamic ankle ligament strains from a computational model driven by motion analysis based kinematic data.

External rotation of the foot has been implicated in high ankle sprains. Recent studies by this laboratory, and others, have suggested that torsional traction characteristics of the shoe-surface interface may play a role in ankle injury. While ankle injuries most often involve damage to ligaments due to excessive strains, the studies conducted by this laboratory and others have largely used surrogate models of the lower extremity to determine shoe-surface interface characteristics based on torque measures alone. The objective of this study was to develop a methodology that would integrate a motion analysis-based kinematic foot model with a computational model of the ankle to determine dynamic ankle ligament strains during external foot rotation. Six subjects performed single-legged, internal rotation of the body with a planted foot while a marker-based motion analysis was conducted to track the hindfoot motion relative to the tibia. These kinematic data were used to drive an established computational ankle model. Ankle ligament strains, as a function of time, were determined. The anterior tibiofibular ligament (ATiFL) experienced the highest strain at 9.2±1.1%, followed by the anterior deltoid ligament (ADL) at 7.8±0.7%, averaged over the six subjects. The peak ATiFL strain occurred prior to peak strain in the ADL in all subjects. This novel methodology may provide new insights into mechanisms of high ankle sprains and offer a basis for future evaluations of shoe-surface interface characteristics using human subjects rather than mechanical surrogate devices.