77 - Ex vivo experimental analysis of the impact of soft tissues on 3D hip joint mobility in two contrasting amniotes

Poster Board Number: 77
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Co-authors:

Stephanie Pierce - Harvard University

Abstract Body : Modern amniotes have a wide range of limb anatomies and postures, yet how this diversity impacts joint function is not well understood. In addition to bone morphology, soft tissues (e.g. integument, muscles, ligaments) are known to permit and constrain joint range of motion (ROM); however, the degree and type of influence imbued by various soft tissue structures remains largely unexplored, especially within a comparative context. Here, we investigate how soft tissues impact 3D hip joint ROM of a sprawling tegu lizard (n = 4) and a parasagittal Virginia opossum (n = 3) by sequentially removing soft tissue layers from cadaveric specimens and using XROMM to quantify joint mobility at each layer. The dataset includes 88,217 marker-tracked bi-planar fluoroscope video frames (i.e. limb “poses”) across four dissected tissue layers for each species: (1) all tissues intact, (2) integument removed, (3) extrinsic hip muscles removed, and (4) intrinsic hip muscles removed. Our results reveal that the tegu has a larger 3D hip joint mobility envelope than the opossum, achieving more poses across most tissue layers (tegu pose volume: layer 1: 156,817 deg³, layer 2: 240,762 deg³, layer 3: 251,395 deg³, and layer 4: 359,604 deg³; opossum pose volume: layer 1: 14,913 deg³, layer 2: 16,113 deg³, layer 3: 95,617 deg³, layer 4: 378,060 deg³). Intrinsic hip muscles play the largest role in constraining ROM in both species, greatly expanding total hip joint mobility when removed, especially in the opossum. Extrinsic hip muscles negligibly constrain mobility in the tegu but have a more moderate influence in the opossum, while the integument has the opposite effect in each species. The type of mobility constraint also differs between species, with soft tissue removal primarily expanding protraction-depression ROM in the tegu, and retraction-elevation ROM as well as long-axis rotation in the opossum. Interestingly, opossum hip joint mobility includes highly coupled rotations when soft tissues are intact and more overall long-axis ROM across tissue layers than the tegu. Further, the opossum hip joint ROM envelope is in a distinct region of 3D mobility space from the tegu hip joint ROM envelope. Taken together, these results suggest that soft tissues can greatly influence the degree and type of joint mobility but that these effects may not be consistent across animals with different limb anatomies and postures.