Application of a Novel Measure of In Vivo Knee Joint Laxity.

Current measures of knee joint laxity, such as those found clinically using the KT-2000 arthrometer, are not highly repeatable or reliable by Huber et al. (1997, "Intratester and Intertester Reliability of the KT-1000 Arthrometer in the Assessment of Posterior Laxity of the Knee," Am. J. Sports Med., 25(4), pp. 479-485). In this study, a noninvasive in vivo magnetic resonance (MR) imaging-based measure of laxity, the knee loading apparatus (KLA) with anterior positioning frame, was evaluated with five normal subjects (repeatability study, n = 3). Effects of hormones and muscle guarding were considered. When compared to the KT-2000, the KLA was found to be more precise (±0.33 mm versus ±1.17 mm) but less reliable (Cronbach's alpha > 0.70 in 0/8 versus 5/8 load levels). Improved control of the initial subject position is recommended for future design iterations. The KLA shows promise as an accurate and reliable tool for measuring in vivo joint and ligament laxity.

Analysis techniques for congruence of the patellofemoral joint.

Quantifying joint congruence may help to understand the relationship between joint function and health. In previous studies, a congruence index (CI) has been used to define subject-specific joint congruence. However, the sensitivity of the CI algorithm to surface representation was unknown. The purpose of this study was to assess the effects of applying five modifications (M1-M5) to the CI algorithm to determine whether the magnitude and variability of the patellofemoral CI is dependent on the surface representation used. The five modifications focused on calculating the CI based on the principal curvature (M1) at the centroid of the contact region, (M2) using an root mean square value for the contact region, (M3) using a mean value for the contact region, (M4) using all digitized points of the patellar surface, and (M5) using all digitized points in contact. The CI found using the contact area (M1, M2, M3, and M5) provides a local measure for congruence, which was shown to increase (decreasing CI) with increasing joint angle. In ten healthy subjects measured with magnetic resonance (MR) images, the patellofemoral joint became significantly more congruent as the knee angle increased from 15 deg to 45 deg using method M5. The magnitude and variability of the patellofemoral CI was dependent on the surface representation used, suggesting that standardization of the surface representation is important to provide a consistent measure. Specifically, M5 provides a local measure of joint congruence, which can account for joint position and orientation. M5 balances the ability to detect differences in congruence between knee angles without introducing high variability.

Application of the advanced system for implant stability testing (ASIST) to natural teeth for noninvasive evaluation of the tooth root interface.

In this paper we present the development of the Advanced System for Implant Stability Testing (ASIST) for application to natural teeth. The ASIST uses an impact measurement combined with an analytical model of the system and surrounding support to provide a measure of the interface stiffness. In this study, an analytical model is developed for a single-rooted natural tooth allowing the ASIST to estimate the stiffness characteristics of the periodontal ligament (PDL). The geometry and inertia parameters of the tooth model are presented in two ways: (1) using full CT scans of the individual tooth and (2) using an approximate geometry model with estimates of only the tooth length and diameter. The developed system is evaluated with clinical data for patients undergoing orthodontic treatment. This study shows that ASIST technique can be applied to natural teeth to estimate the stiffness characteristics of the PDL. The developed system can provide a valuable clinical tool for assessment of tooth stability properties and PDL stiffness in a variety of clinical situations such as dental trauma, orthodontics, and periodontology.

Advanced System for Implant Stability Testing (ASIST).

This study presents the Advanced System for Implant Stability Testing (ASIST) which provides a non-invasive, quantitative measure of the stability of percutaneous implants used for craniofacial rehabilitation such as bone anchored hearing aids or dental implants. The ASIST uses an impact technique coupled with an analytical model which allows the measure to be independent of the system components. This paper presents a laboratory evaluation of the ASIST for the Oticon Medical Ponto and the Cochlear Baha Connect bone anchored hearing aid (BAHA) systems. There is minimal effect of abutment length on the ASIST Stability Coefficient (ASC) value, indicating that the method is able to isolate the interface properties from the overall system and the measurement is independent of attached components. Additionally, the ASIST was able to detect differences between different implant installations suggesting that it may be sensitive to changes in interface stiffness.

Quantifying in vivo laxity in the anterior cruciate ligament and individual knee joint structures.

A custom knee loading apparatus (KLA), when used in conjunction with magnetic resonance imaging, enables in vivo measurement of the gross anterior laxity of the knee joint. A numerical model was applied to the KLA to understand the contribution of the individual joint structures and to estimate the stiffness of the anterior-cruciate ligament (ACL). The model was evaluated with a cadaveric study using an in situ knee loading apparatus and an ElectroForce test system. A constrained optimization solution technique was able to predict the restraining forces within the soft-tissue structures and joint contact. The numerical model presented here allowed in vivo prediction of the material stiffness parameters of the ACL in response to applied anterior loading. Promising results were obtained for in vivo load sharing within the structures. The numerical model overestimated the ACL forces by 27.61-92.71%. This study presents a novel approach to estimate ligament stiffness and provides the basis to develop a robust and accurate measure of in vivo knee joint laxity.

Differences in patellofemoral contact mechanics associated with patellofemoral pain syndrome.

Patellofemoral pain syndrome (PFPS) is a disorder of the patellofemoral (PF) joint in which abnormal tracking is often cited as a factor in pain development. PF tracking is partially dependent on passive stabilizers (ex: PF geometry). Relations amongst PFPS, PF tracking, and contact mechanics are poorly understood. In-vivo investigation of passive PF joint stabilizers including PF tracking, contact mechanics, cartilage thickness, and patellar shape will allow structural characterization of the PF joint and may highlight differences associated with PFPS. This study examined the role that passive stabilizers play in PFPS (n=10) versus healthy subjects (n=10). PF tracking (contact area centroid migration), cartilage thickness, shape, congruence, and contact patterns were quantified using magnetic resonance imaging during isometric loading at 15 degrees , 30 degrees , and 45 degrees of knee flexion. Distinct relationships were identified between patellar shape and tracking and contact, particularly at low flexion (15-30 degrees ). Healthy subjects exhibited distinct PF tracking and contact patterns related to Type I patella shape (80%) with increasing total contact area (p<0.001) and proximal centroid migration (15-30 degrees p=0.012; 30-45 degrees p<0.001) for increasing knee angles. PFPS subjects deviated from these patterns at low flexion, demonstrating higher total contact area than healthy subjects (p=0.046 at 15 degrees ), lack of proximal centroid migration (15-30 degrees ), and more Type II (30%) and III (20%) patella shapes. This study highlights a new finding that patellar shape combined with low degrees of flexion (15-30 degrees ) may be important to consider, as this is where PFPS tracking and contact patterns deviate from healthy.