The kinematic and kinetic characteristics of the knee after TKR are known to be strongly influenced by the alignment and positioning of the implanted components. In this paper we apply a virtual multi-fiber ligament model to a rigid body model of the post-surgical knee to explore how variations in alignment and positioning affect the predicted behavior of the ligaments and contact forces. We vary the angular and translational positioning of the femoral and tibial TKR components relative to the bone. Meanwhile the proximal and distal insertion sites of the ligaments are held constant relative to the bony structures. We evaluate sensitivity of the ligament balance and peak ligament tension through the passive flexion arc in response to the variation in positioning and alignment of the TKR components. With further development, this work holds the promise of applications in surgical planning and virtual arthroplasty.

Modeling the kinetic effects of the soft tissue structures is a major challenge for dynamic simulation of knees and other joints. We describe a technique whereby a multi-fiber ligament model is evolved to reproduce accurately the passive kinetics of a knee joint. The passive motion can be derived from patient-specific motion capture data. It may also be derived in-silico from a desired articular surface geometry, for example an implant or a surface model acquired by radiography. The technique operates by optimizing the tibial ligament insertion sites to minimize the change in strain energy through a specified range of motion. It is believed that the ligament model so produced is valuable for loaded kinetic and kinematic joint studies as well. The results therefore may be used to inform implant positioning during surgical planning.

The Grood and Suntay coordinate system is a well-known framework for defining relative joint motions referenced to clinically meaningful anatomical directions. However, in general the Grood and Suntay unit vectors do not intersect at a point, and the “floating” (second) unit vector does not have a fixed location relative to the joint. These characteristics can introduce complications when analyzing joint forces such as the forces resulting from contact or from soft tissue structures. We have developed a methodology to address these issues by resolving forces along directions that intersect at a point fixed to one of the joint bodies. The work is demonstrated using Vivo Sim Control and Vivo Sim Visualization software.

The Vivo joint motion simulator, Figure 1, and Vivo Sim Visualization software were developed to investigate joint dynamics. They use the Grood and Suntay coordinate system. Figure 2, produced using Vivo Sim Visualization, shows a solid-body model of a knee, with Grood and Suntay frames in red and green. The light blue lines are a partial soft tissue model. Figure 3 is a representation of the Grood and Suntay coordinate system for a joint set in an arbitrary pose. Figure 3 shows the primary and secondary Grood and Suntay coordinate frames, labeled “F” and “T” (light arrows), the unit vectors e1, e2 and e3 (short dark arrows), and the displacement coordinates q1, q2, q3, α, β and γ. In general, there is no point in space about which there is zero net moment resulting from forces acting collinearly with the unit vectors. In order to obtain a zero-moment reaction condition, compensating moments must be computed and applied. In addition, because e2 floats, force collinear with that vector has a line of action that translates relative to the joint. These issues are inconvenient from an analytical standpoint and could be confusing for practitioners.

In Vivo's control system and in the Vivo Sim Visualization software, commanded joint forces and moments are resolved to axes parallel to the e1, e2 and e3 unit vectors but passing through the T origin. The origin of T is chosen to lie approximately collinear with the resultant of contact forces. A multi-fiber ligament model calculates forces collinear with the ligament fibers, which are resolved to equivalent forces and moments acting through the origin of T.

With this methodology, forces along anatomically-meaningful directions can be applied to or reported from the joint without the need to compute compensating moments. The lines of action of these forces can change orientation according to joint movements, but they always pass through a point fixed to the second body. We have implemented this methodology in the Vivo Joint Simulator and the Vivo Sim Visualization software.

With the boom in metal-on-metal hip resurfacing and hard-on-hard total hip replacements (THRs) with extremely low wear, accurate tribological measurements become difficult. Characterizing THR friction can help in this, especially if the progress of such friction can be tracked during wear tests. Friction measurement can also be used as a tool to study the effects of acetabular-liner deformation during insertion, and possible femoral head “clamping”. This study presents estimates of friction during extended wear testing on THRs of the same size but with different material combinations, using a technique (previously introduced) based on equilibrium of forces and moments measured in the simulator.

All tests were based on five million cycles (Mc) and samples of size-44mm (head diameter). Samples included 6 metal-on-UHMWPE (MOP) (3 with conventional UHMWPE and 3 with highly-cross-linked (HXL) UHWMPE liners), 6 metal-on-metal (MOM) (3 TiN-coated and 3 uncoated), 6 MOM resurfacing (3 standard and 3 with small pockets for lubrication transport), and 3 ceramic-on-UHMWPE (COP) THRs (MOM resurfacing and COPs for 2Mc only). All were lubricated with diluted bovine serum with 20g/l protein concentration at 37°C, and subjected to the loading and rotations of the walking cycle in ISO-14242-1 on a twelve-station hip simulator (AMTI, Boston).

The conventional and HXL MOPs had steady friction factors of 0.045±0.009 and 0.046±0.003 over 5Mc, explained by the stability of wear rates of both these MOP types (72.0±2.81mg/Mc and 14.2±3.57mg/Mc, respectively). However, during the “bedding-in” period (first 0.5Mc), the conventional MOP friction factor rose from 0.047±0.004 to 0.057±0.004 while high wear was occurring (147.1±10.08mg/Mc). The TiN-coated and uncoated MOMs displayed initial friction factors of 0.124±0.117 and 0.039±0.003 respectively. The high standard deviation for the coated THRs was due to coating removal on one specimen which caused scratches and scuffs on its articulating surfaces. This specimen had a friction factor of 0.260 at 0.028Mc. By 1Mc, the TiN coating wore away on the other two coated specimens (friction factors at 1Mc: coated 0.081±0.036, uncoated 0.050±0.014). Over the 5Mc test, average friction factors for the coated and uncoated THRs were 0.097±0.020 and 0.049±0.014 respectively. The 44mm standard and “pocketed” MOM resurfacing THRs displayed initial friction factors of 0.038±0.009 and 0.059±0.026 respectively that increased to the same level at 2Mc (0.094±0.020 and 0.094±0.029, respectively). No difference in wear was detected between the two resurfacing head types (wear rates over 2Mc: standard 3.32±0.25mg/ Mc, pocketed 2.22±1.76mg/Mc), but curiously, both types exhibited an equal level of scratching and scuffing on their articular surface. Finally, the three COP THRs exhibited high liner wear over 2Mc (97.44±3.08mg/Mc), which slowed after the “bedding-in” period. The friction factor also decreased from 0.091±0.005 to 0.070±0.008 over the same period as the UHMWPE liner conformed to the ceramic head.

The method utilized here facilitates on-line sampling throughout the progress of a prolonged wear test, and therefore allows predictions on THR performance/wear to be made. When high friction factors were observed, a high wear rate was occurring and measured on the THR specimens, or damage to articulating surfaces was seen.

There are many types of treatment used to manage the frozen shoulder, but there is no consensus on how best to manage patients with this painful and debilitating condition. We conducted a review of the evidence of the effectiveness of interventions used to manage primary frozen shoulder using the Cochrane Database of Systematic Reviews, the Database of Abstracts of Reviews of Effects, the Physiotherapy Evidence Database, MEDLINE and EMBASE without language or date restrictions up to April 2009. Two authors independently applied selection criteria and assessed the quality of systematic reviews using the Assessment of Multiple Systematic Reviews (AMSTAR) tool. Data were synthesised narratively, with emphasis placed on assessing the quality of evidence.

In total, 758 titles and abstracts were identified and screened, which resulted in the inclusion of 11 systematic reviews. Although these met most of the AMSTAR quality criteria, there was insufficient evidence to draw firm conclusions about the effectiveness of treatments commonly used to manage a frozen shoulder. This was mostly due to poor methodological quality and small sample size in primary studies included in the reviews. We found no reviews evaluating surgical interventions.

More rigorous randomised trials are needed to evaluate the treatments used for frozen shoulder.

Introduction: Intervertebral disc degeneration occurs with ageing and is often associated with back pain. During such degeneration, gross morphological differences between the central nucleus pulposus (NP) and outer annulus fibrosus (AF) are lost and the disc loses hydration and height due to decreased proteoglycan content. The cartilage endplate may also become calcified and this blocks the passage of nutrients into the disc, causing cell death and further degeneration. A potential therapy of degeneration is “re-inflation” of the disc with the use of hydrogels seeded with autologous disc cells. In this study, we have assessed the ability of a variety of hydrogels to support intervertebral disc cell growth.

Method: Intervertebral disc cells were isolated enzymatically from bovine tails and cultured as a monolayer in 10% foetal calf serum in DMEM containing antibiotics and ascorbic acid. This stimulates the cells to proliferate and thereby produces increased cell numbers. The cells were then seeded onto various hydrogels including hyaluronic acid (HA), 2-hydroxyethyl methacrylate (HEMA), N’N’ dimethyl methacrylate (NNDMA) and polyacryloyl morpholine (AMO) before harvesting at set time points of 1, 3, 6 and 9 days for hyaluronic acid and 1, 7, 14, 21, and 28 days for the other hydrogels. Cell number, morphology, viability and adherence to or migration into the hydrogels were assessed. Cell proliferation was also determined by immunostaining for the Ki67 antigen.

Results: Disc cells became incorporated in the HA gel, adopted a spherical morphology and remained viable for up to nine days. However, after a few days, a large proportion of the cells began to migrate through the gel to form a monolayer on the bottom of the tissue culture well. These monolayered cells became fibroblastic and proliferated. NP cells appeared to proliferate to a greater extent than AF cells both in monolayer and in suspension. Ki67 antigen immunostaining confirmed cell proliferation. On the non-porous HEMA, NNDMA and AMO, both cell types adhered and adopted a fibroblast-like morphology. Cell adhesion was greatest to the HEMA. NNDMA and AMO had lower levels of cell adherence. Both cell types became incorporated into the porous materials and adopted a rounded morphology. Cell incorporation appeared to be greatest into porous HEMA.

Conclusion: These initial studies show that intervertebral disc cells will adhere to or migrate into a variety of hydrogels and remain viable. The morphology and proliferative capacity of cells derived from both the AF and NP were responsive to the structure of the hydrogel with which they were cultured. Thus, cells were able to become fibroblastic or chondrocytic. Further analyses will reveal whether matrix synthesis by disc cells is similarly responsive to the hydrogel format. The results of these experiments suggest that the hydrogels tested have potential as support matrices in intervertebral disc repair to provide relief from discogenic low-back pain.