Purpose: The mechanical function and strain behavior of the knee meniscus is not fully understood, due to multiple tissues with disparate properties, as well as complex contact patterns and intricate loading mechanisms. More comprehensive understanding of joint mechanics may contribute to improved treatment options for patients with injuries and osteoarthritis. There is very limited information available on the 3D strain of the intact meniscus. The objective of this work was to use mCT with copper microsphere markers to quantify three-dimensional strain of the meniscus under physiologic loading.

Method: Two healthy fresh frozen ovine knee specimens were harvested. Copper microspheres (0.5mm) were injected into anterior and posterior tetrahedral clusters in the medial meniscus using 20-gauge hypodermic needles. Needle cavities were sealed with ovine tendon tissue. Joints were loaded to 100% body weight in a 4 DOF CT-compatible pneumatically-driven device with flexion angles ranging from 62–98°. Images were acquired with an eXplore Locus Ultra mCT scanner and reconstructed with commercial software. A time series of images were acquired with the joint unloaded, during static loading, and at a reduced load (25% BW).

Results: The average maximum principle strains in the anterior element of the two specimens at 62o of flexion increased by 21% during loading and decreased by 13% during unloading. The maximum principle strains were 28% larger in the anterior element than the posterior. The strains in the anterior element decreased by 6.5% with time following load application, and decreased by 16% with load reduction, yielding relatively low residual strain. Strains were 2% larger in the anterior portion with larger flexion angles.

Conclusion: The objective of this work was to develop a reliable method for quantifying 3D strains in the meniscus. Results support the notion that mCT imaging with copper microspheres in the meniscus may be a viable technique for more comprehensive 3D strain analysis. The relatively low residual strains measured in this study indicate that copper microspheres are stable markers in this application. This technique may be useful in directing future studies aimed at understanding the impact of meniscal pathologies and the success of repair techniques.

Stress shielding (i.e. reduction in bone strains) in the distal ulna is commonly noted following ulnar head replacement arthroplasty. Optimal design parameters for distal ulnar implants, including the length of the stem, are currently unknown. The purpose of this study was to investigate the effect of stem length on bone strains along the length of the ulna.

Strain gauges were applied to each of eight cadaveric ulnae to measure bending loads at six locations along each ulna’s length (approximately 1.5, 2.5, 4.0, 6.0, 8.0, and 13.0cm from the ulnar head). The proximal portion of each bone was secured in a custom-designed jig. A materials testing machine applied loads (5–30N) to the ulnar head while native strains were recorded. The ulnar head was removed and the loading procedure repeated for cemented stainless steel stems 3 and 7cm in length, according to a previously reported technique (Austman et al, CORS 2006). Other stem lengths between 3 and 7cm were tested in 0.5cm intervals with a 20N load applied only. Data were analyzed using a two-way repeated measures ANOVA (á=0.05).

In general, distal bone strains increased as stem length decreased (e.g. average microstrains at the second distal-most gauges: 138±13 (7cm), 147±15 (6cm), 159±21 (5cm), 186±40 (4cm), 235±43 (3cm)). The native strains were different from all stem lengths for the four distal-most gauges (p< 0.05). No differences were found between any stem length and the native bone at the two proximal-most gauges. The 3cm stem replicated the native strains more closely than the 7cm, over all applied loads (e.g. average microstrains at the third gauge level for a 25N load: 357±59 (native), 396±74 (3cm), 257±34 (7cm)).

No stem length tested matched the native strains at all gauge locations. The 3cm stem results were closer to the native strains than the 7cm stem for all loads at gauges overtop of the stem. Overall, the 3cm stem produced the highest strains, and thus would likely result in less distal ulnar bone resorption after implantation. These results suggest that shorter (approximately 3cm) stems should be considered for distal ulnar implants to potentially reduce stress shielding, although this must be balanced by adequate stem length for fixation.