Derek C. Wilson

Localized cartilage assessment with three-dimensional dGEMRIC in asymptomatic hips with normal morphology and cam deformity

BACKGROUND Cam deformities cause femoroacetabular impingement and damage the acetabular labral-chondral complex. The aims of this study were to investigate the potential of delayed gadolinium-enhanced magnetic resonance imaging of cartilage (dGEMRIC) to detect cartilage disease in asymptomatic hips with cam deformities compared with morphologically normal hips, establish whether dGEMRIC could identify advanced disease in hips with positive clinical findings, and establish whether cartilage damage correlated with the severity of the cam deformity. METHODS Subjects were recruited from a prospective study of individuals with a family history of osteoarthritis and their spouses who served as control subjects. Their symptoms and impingement test results were recorded. Asymptomatic hips with normal radiographic joint-space width were placed in a subgroup according to the presence of a cam deformity and the impingement test result. dGEMRIC was performed on a 3-T system, studying two regions of interest: the anterosuperior aspect of the acetabular cartilage (T1(acet)) and the total femoral and acetabular cartilage (T1(total)). The ratio T1(acet)/T1(total) gave the relative glycosaminoglycan content in the anterosuperior aspect of the acetabular cartilage. The cohort was placed in subgroups by joint morphology, impingement test status, and genetic predisposition; the mean T1 scores were compared, and the alpha angle and T1 were correlated. RESULTS Of thirty-two subjects (mean age, fifty-two years), nineteen had cam deformities. Hips with a cam deformity had reduced acetabular glycosaminoglycan content compared with normal hips (mean T1(acet)/T1(total), 0.949 and 1.093, respectively; p = 0.0008). Hips with a positive impingement test result had global depletion of glycosaminoglycan compared with hips with a negative result (mean T1(total), 625 ms versus 710 ms; p = 0.0152). T1(acet) inversely correlated with the magnitude of the alpha angle (r = -0.483, p = 0.0038), suggesting that the severity of cartilage damage correlates with the magnitude of the cam deformity. All of these differences occurred irrespective of genetic predisposition. CONCLUSIONS The dGEMRIC technique can detect cartilage damage in asymptomatic hips with cam deformities and no radiographic evidence of joint space narrowing. This damage correlates with cam deformity severity. Further study of the application of dGEMRIC as an imaging biomarker of early osteoarthritis is justified to validate its prognostic accuracy, identify subjects for clinical trials, and evaluate the effectiveness of surgical procedures.

The effect of dynamic posterior stabilization on facet joint contact forces: an in vitro investigation.

Study Design. Facet contact forces in the lumbar spine were measured during flexibility tests using thin film electroresistive sensors in intact cadaveric spine specimens and in injured specimens stabilized with a dynamic posterior system. Objective. The purpose of this study was to investigate the effect of the Dynesys system on the loading in the facet joints. Summary of Background Data. The Dynesys, a posterior nonfusion device, aims to preserve intersegmental kinematics and reduce facet loads. Recent biomechanical evidence showed that overall motion is less with the Dynesys than in the intact spine, but no studies have shown its effect on facet loads. Methods. Ten human cadaveric lumbar spine specimens (L2–L5) were tested by applying a pure moment of ±7.5 N m in 3 directions of loading with and without a follower preload of 600 N. Test conditions included an intact specimen and an injured specimen stabilized with 3 Dynesys spacer lengths. Bilateral facet contact forces were measured during flexibility tests using thin film electroresistive sensors (Tekscan 6900). Results. Implanting the Dynesys significantly increased peak facet contact forces in flexion (from 3 N to 22 N per side) and lateral bending (from 14 N to 24 N per side), but had no significant effect on the magnitude of the peak forces in extension and axial rotation. Peak facet loads were significantly lower with the long spacer compared with the short spacer in flexion and lateral bending. Conclusion. Implantation of the Dynesys did not affect peak facet contact forces in extension or axial rotation compared with an intact specimen, but did alter these loads in flexion and lateral bending. The spacer length affected the compression of the posterior elements, with a shorter spacer typically producing greater facets loads than a longer one.

Can extra-articular strains be used to measure facet contact forces in the lumbar spine? An in-vitro biomechanical study

Abstract Experimental measurement of the load-bearing patterns of the facet joints in the lumbar spine remains a challenge, thereby limiting the assessment of facet joint function under various surgical conditions and the validation of computational models. The extra-articular strain (EAS) technique, a non-invasive measurement of the contact load, has been used for unilateral facet joints but does not incorporate strain coupling, i.e. ipsilateral EASs due to forces on the contralateral facet joint. The objectives of the present study were to establish a bilateral model for facet contact force measurement using the EAS technique and to determine its effectiveness in measuring these facet joint contact forces during three-dimensional flexibility tests in the lumbar spine. Specific goals were to assess the accuracy and repeatability of the technique and to assess the effect of soft-tissue artefacts. In the accuracy and repeatability tests, ten uniaxial strain gauges were bonded to the external surface of the inferior facets of L3 of ten fresh lumbar spine specimens. Two pressure-sensitive sensors (Tekscan) were inserted into the joints after the capsules were cut. Facet contact forces were measured with the EAS and Tekscan techniques for each specimen in flexion, extension, axial rotation, and lateral bending under a ±7.5 N m pure moment. Four of the ten specimens were tested five times in axial rotation and extension for repeatability. These same specimens were disarticulated and known forces were applied across the facet joint using a manual probe (direct accuracy) and a materials-testing system (disarticulated accuracy). In soft-tissue artefact tests, a separate set of six lumbar spine specimens was used to document the virtual facet joint contact forces during a flexibility test following removal of the superior facet processes. Linear strain coupling was observed in all specimens. The average peak facet joint contact forces during flexibility testing was greatest in axial rotation (71±25 N), followed by extension (27±35 N) and lateral bending (25±28 N), and they were most repeatable in axial rotation (coefficient of variation, 5 per cent). The EAS accuracy was about 20 per cent in the direct accuracy assessment and about 30 per cent in the disarticulated accuracy test. The latter was very similar to the Tekscan accuracy in the same test. Virtual facet loads (r.m.s.) were small in axial rotation (12 N) and lateral bending (20 N), but relatively large in flexion (34 N) and extension (35 N). The results suggested that the bilateral EAS model could be used to determine the facet joint contact forces in axial rotation but may result in considerable error in flexion, extension, and lateral bending.

Multi-Contrast MR for Enhanced Bone Imaging and Segmentation

Musculoskeletal applications of MRI are increasing rapidly but a major challenge for researchers is the ability to efficiently and accurately segment structures of interest, such as bone, which is typically required to perform further quantitative analyses. Manual tracing is extremely time consuming and introduces problematic user variability. Automated segmentation is usually preferred; however, the accuracy and robustness of current methods still suffer from significant limitations. In this paper, we propose a novel approach for simplifying such segmentation tasks by optimizing MR protocols specifically for bone data acquisition. We present multi-contrast MR bone data acquired using short-TR T1W and fat suppression scans and demonstrate how this data can be used within an automated segmentation framework in order to improve accuracy of bone segmentation. Validation was performed on knee joint data with quantitative segmentation results on our multi- contrast data showing superior performance compared to results obtained using conventional single-contrast data. Improvements in contrast to noise ratio of 39.24 and in sensitivity and specificity of 4.09% and 4.17%, respectively, for the tibia, and 4.4% and 5.74% for the femur, were achieved.

An ex vivo biomechanical comparison of a novel vertebral compression fracture treatment system to kyphoplasty

BACKGROUND Vertebral compression fracture repair aims to relieve pain and improve function by restoring vertebral structure and biomechanics, but is still associated with risks arising from polymethylmethacrylate cement extravasation. The Kiva® Vertebral Compression Fracture Treatment System, a stacked coil implant made of polyetheretherketone and delivered over a guide-wire, is a novel device designed to provide height restoration and mechanical stabilization, while improving cement containment and minimizing disruption of cancellous bone. The objective of this study was to determine whether the Kiva system is as effective as balloon kyphoplasty at restoring mechanical properties in osteoporotic vertebral compression fractures. METHODS Wedge fractures were created in the middle vertebra of fourteen osteoporotic three-vertebra spine segments and then repaired with either the Kiva or kyphoplasty procedure. Height, stiffness and displacement under compression of the spine segments were measured for four conditions: intact, fractured, augmented, and post-cyclic eccentric loading (50,000cycles, 200-500N, 30mm anterior lever arm). FINDINGS No significant differences were seen between the two procedures for height restoration, stiffness at high or low loads, or displacement under compression. However, the Kiva System required an average of 66% less cement than kyphoplasty to achieve these outcomes (mean 2.6 (SD 0.4) mL v. mean 7.5 (SD 0.8) mL 0; P<0.01). Extravasations and excessive posterior cement flow were also significantly lower with Kiva (0/7 v. 4/7; P<.05). INTERPRETATION Kiva exhibits similar biomechanical performance to balloon kyphoplasty, but may reduce the risk of extravasation through the containment mechanism of the implant design and by reducing cement volume.

Accuracy of cut-off acetabular reamers for minimally invasive THA.

Cut-off reamers have been introduced for minimally invasive hip replacement to make reamer insertion through the small incision easier. However, the accuracy of cut-off reamers in comparison to traditional hemispherical reamers has not been documented. We reamed four human cadaveric hips using a cut-off reamer and three hips using a standard reamer. We started with smallest size reamer to remove subchondral bone, and the size was progressively increased until breaching the acetabular floor. We performed computed tomography scans for each reamer size to digitally determine the true dimensions and sphericity of the reamed acetabula. The cut-off reamers breached the acetabulum at a smaller size than with a standard reamer in two specimens, and at the same size as the standard reamer in one specimen. The accuracy of each reamer size was determined by quantifying the percentage of the reamed acetabular surface that was within 0.5 mm of the hemispherical reamer size. The average accuracy of the cut-off reamers was 70% compared with 81% for the standard reamers. The cut-off acetabular reamers showed a trend toward decreased accuracy that may be attributable to a tendency of the reamer to wobble in use.