The relationship between knee arthroplasty and foot loading.

Surgeons have questioned whether foot deformity applies abnormal loading on a knee implant. A total of 24 patients with mild knee deformity underwent a static recording of foot loading prior to and at 3 months following knee replacement. Of these patients, 13 had a preoperative varus deformity. The recorded postoperative to preoperative loading in all 6 geographic sites was decreased by an average of 10%. The largest changes were observed in the hallux and lesser toe masks, whereas the postoperative to preoperative foot pressure ratio in the metatarsal head (lateral and medial), heel, and midfoot masks was 0.94. This preliminary investigation reveals a minimal change in geographic foot loading following total knee arthroplasty in patients with mild knee deformity.

Effect of increasing implant height on lumbar spine kinematics and foraminal size using the ProDisc-L prosthesis.

STUDY DESIGN: A biomechanical study using human lumbar spines. OBJECTIVE: To test the hypotheses that with increasing implant height (1) the range of motion (ROM) of the implanted segment will decrease, (2) the segmental lordosis will increase, and (3) the size of the neural foramens will increase. SUMMARY OF BACKGROUND DATA: Little is known about the effects of the implant height on the segmental motion and foraminal size at the implanted level. METHODS: Seven human lumbar spines (age, 54.4+/-11.4 years; L1-sacrum) were tested intact, and after discectomy at L4-L5 and sequential insertion of ProDisc-L implants (Synthes Spine, Paoli, PA) of increasing heights (10, 12, and 14 mm). The specimens were tested in flexion (8 Nm) and extension (-6 Nm) with a 400 N follower preload as well as in lateral bending (+/-6 Nm) and axial rotation (+/-5 Nm) without preload. Three-dimensional motions were measured at L4-L5. Foraminal sizes at L4-L5 were measured in the specimen's neutral posture under a 400 N preload for the intact spine and after each implantation using finely graded cylindrical probes. Segmental lordosis was measured in the specimen's neutral posture under a 400 N preload by analyzing digital fluoroscopic images. Effects of implant height on the kinematics, foraminal size, and segmental lordosis were assessed using paired comparisons with Bonferroni correction. RESULTS: Increasing implant height from 10 mm to 14 mm caused a significant decrease (P<0.05) in segmental ROM by up to 37%+/-21% in flexion/extension, 33%+/-18% in lateral bending, and 29%+/-28% in axial rotation. Increasing implant height also produced a significant increase in segmental lordosis (P<0.05): from 9.7 degrees+/-2.9 degrees at 10 mm, to 16.1 degrees+/-5.1 degrees at 14 mm. The increase in foraminal size, while significant, was only 4.6%+/-3.2% when comparing 10 mm to 14 mm implants. CONCLUSION: These results suggest that a smaller implant height should be selected to optimize the ROM of the implanted segment and maintain sagittal balance.

Static measure of foot loading.

Static and dynamic loading of the foot are important characteristics for understanding human walking in both health and disease. The goal of this investigation was to determine whether an objective measure of normal midstance loading of the foot could reliably be recorded using readily available disposable qualitative recording devices. Ten randomly selected normal volunteers were trained to step on Harris mat and Pressure Stat recording devices during normal walking. Each of the recordings was divided into 5 weight-bearing regions by 2 separate examiners. After outlining each foot, the recordings were digitized and compared. Interobserver reliability ranged from 0.81 to 0.96 for the Harris mat technique and 0.94 to 0.97 for the Pressure Stat technique. Data from a linear regression plot indicate high precision of calculations of the foot masks between the 2 examiners based on an R(2) value of 0.966 using the Pressure Stat method. These data plus a linear regression plot suggest that both qualitative recording devices, when digitized using a standardized format, appear to obtain a reliable objective measure of midstance loading during normal gait. The Pressure Stat device may be slightly more reliable. It is planned to use this standardized experimental model to compare objectively patterns of midstance loading in patients with injury or disease that is capable of altering normal walking.

Altered disc pressure profile after an osteoporotic vertebral fracture is a risk factor for adjacent vertebral body fracture.

This study investigated the effect of endplate deformity after an osteoporotic vertebral fracture in increasing the risk for adjacent vertebral fractures. Eight human lower thoracic or thoracolumbar specimens, each consisting of five vertebrae were used. To selectively fracture one of the endplates of the middle VB of each specimen a void was created under the target endplate and the specimen was flexed and compressed until failure. The fractured vertebra was subjected to spinal extension under 150 N preload that restored the anterior wall height and vertebral kyphosis, while the fractured endplate remained significantly depressed. The VB was filled with cement to stabilize the fracture, after complete evacuation of its trabecular content to ensure similar cement distribution under both the endplates. Specimens were tested in flexion-extension under 400 N preload while pressure in the discs and strain at the anterior wall of the adjacent vertebrae were recorded. Disc pressure in the intact specimens increased during flexion by 26 +/- 14%. After cementation, disc pressure increased during flexion by 15 +/- 11% in the discs with un-fractured endplates, while decreased by 19 +/- 26.7% in the discs with the fractured endplates. During flexion, the compressive strain at the anterior wall of the vertebra next to the fractured endplate increased by 94 +/- 23% compared to intact status (p < 0.05), while it did not significantly change at the vertebra next to the un-fractured endplate (18.2 +/- 7.1%, p > 0.05). Subsequent flexion with compression to failure resulted in adjacent fracture close to the fractured endplate in six specimens and in a non-adjacent fracture in one specimen, while one specimen had no adjacent fractures. Depression of the fractured endplate alters the pressure profile of the damaged disc resulting in increased compressive loading of the anterior wall of adjacent vertebra that predisposes it to wedge fracture. This data suggests that correction of endplate deformity may play a role in reducing the risk of adjacent fractures.