No abstract

The transverse carpal ligament (TCL), the main part of the flexor retinaculum, serves as an anchor for the thenar muscles: abductor pollicis brevis (APB), superficial head of the flexor pollicis brevis (sFPB), and opponens pollicis (OPP). Biomechanically, the thenar muscles rely on their TCL anchoring to transmit muscle contractions distally for thumb force and motion production, and reciprocally, muscle contraction interacts with the TCL at the proximal end through the origins. However, scarce knowledge exists regarding the distribution pattern of the thenar muscle origins. The purpose of this study was to understand the anatomical interface between the thenar muscles and TCL by examining the origin distributions of the individual muscles. Ten cadaveric specimens were dissected for digitization of the muscle origins and TCL volar surface. Digitized data were used for mesh reconstruction and calculation of surface areas and centroids. The origin areas for APB, sFPB, and OPP were 105.8 ± 30.3, 64.6 ± 15.2, and 245.9 ± 70.7 mm 2 , respectively. The surface area of the TCL was 386.2 ± 86.9 mm 2. The origin areas of APB and OPP on the TCL were comparable, 18.4 ± 4.8% and 17.3 ± 9.6% of the TCL area, respectively. The origin locations for APB, sFPB, and OPP were in proximal-radial quadrant of the TCL, on distal aponeurosis outside the TCL, and around the ridge of trapezium, respectively. The knowledge of the anatomical interface provides a foundation for the understanding of biomechanical interactions between the muscles and ligaments and pathomechanical implications.

OBJECTIVE Excessive stress and motion at the L5–S1 level can lead to degenerative changes, especially in patients with posterior instrumentation suprajacent to L5. Attention has turned to utilization of L5–S1 anterior lumbar interbody fusion (ALIF) to stabilize the lumbosacral junction. However, questions remain regarding the effectiveness of stand-alone ALIF in the setting of prior posterior instrumented fusions terminating at L5. The purpose of this study was to assess the biomechanical stability of an L5–S1 ALIF with increasing lengths of posterior thoracolumbar constructs. METHODS Seven human cadaveric spines (T9–sacrum) were instrumented with pedicle screws from T10 to L5 and mounted to a 6 degrees-of-freedom robot. Posterior fusion construct lengths (T10–L5, T12–L5, L2–5, and L4–5) were instrumented to each specimen, and torque-fusion level relationships were determined for each construct in flexion-extension, axial rotation, and lateral bending. A stand-alone L5–S1 ALIF was then instrumented, and L5–S1 motion was measured as increasing pure moments (2 to 12 Nm) were applied. Motion reduction was calculated by comparing L5–S1 motion across the ALIF and non-ALIF states. RESULTS The average motion at L5–S1 in axial rotation, flexion-extension, and lateral bending was assessed for each fusion construct with and without ALIF. After adding ALIF to a posterior fusion, L5–S1 motion was significantly reduced relative to the non-ALIF state in all but one fused surgical condition (p < 0.05). Longer fusions with ALIF produced larger L5–S1 motions, and in some cases resulted in motions higher than native state motion. CONCLUSIONS Posterior fusion constructs up to L4–5 could be appropriately stabilized by a stand-alone L5–S1 ALIF when using a nominal threshold of 80% reduction in native motion as a potential positive indicator of fusion. The results of this study allow conclusions to be drawn from a biomechanical standpoint; however, the clinical implications of these data are not well defined. These findings, when taken in appropriate clinical context, can be used to better guide clinicians seeking to treat L5–S1 pathology in patients with prior posterior thoracolumbar constructs.

OBJECTIVE The direct lateral approach is an alternative to the transoral or endonasal approaches to ventral epidural lesions at the lower craniocervical junction. In this study, the authors performed, to their knowledge, the first in vitro biomechanical evaluation of the craniovertebral junction after sequential unilateral C1 lateral mass resection. The authors hypothesized that partial resection of the lateral mass would not result in a significant increase in range of motion (ROM) and may not require internal stabilization. METHODS The authors performed multidirectional in vitro ROM testing using a robotic spine testing system on 8 fresh cadaveric specimens. We evaluated ROM in 3 primary movements (axial rotation [AR], flexion/extension [FE], and lateral bending [LB]) and 4 coupled movements (AR+E, AR+F, LB + left AR, and LB + right AR). Testing was performed in the intact state, after C1 hemilaminectomy, and after sequential 25%, 50%, 75%, and 100% C1 lateral mass resection. RESULTS There were no significant increases in occipital bone (Oc)–C1, C1–2, or Oc–C2 ROM after C1 hemilaminectomy and 25% lateral mass resection. After 50% resection, Oc–C1 AR ROM increased by 54.4% (p = 0.002), Oc LB ROM increased by 47.8% (p = 0.010), and Oc–C1 AR+E ROM increased by 65.8% (p < 0.001). Oc–C2 FE ROM increased by 7.2% (p = 0.016) after 50% resection; 75% and 100% lateral mass resection resulted in further increases in ROM. CONCLUSIONS In this cadaveric biomechanical study, the authors found that unilateral C1 hemilaminectomy and 25% resection of the C1 lateral mass did not result in significant biomechanical instability at the occipitocervical junction, and 50% resection led to significant increases in Oc–C2 ROM. This is the first biomechanical study of lateral mass resection, and future studies can serve to validate these findings.

This study investigated the biomechanical effects of thenar muscles (abductor pollicis brevis, APB; superficial head of flexor pollicis brevis, sFPB; opponens pollicis, OPP) on the transverse carpal ligament formed carpal arch under force application by individual or combined muscles (APB, sFPB, OPP, APB-sFPB, sFPB-OPP, APB-OPP, and APB-sFPB-OPP). In ten cadaveric hands, thenar muscles were loaded under 15% of their respective maximal force capacity, and ultrasound images of the cross section of the distal carpal tunnel were collected for morphometric analyses of the carpal arch. The carpal arch height and area were significantly dependent on the loading condition (p &lt; 0.01), muscle combination (p &lt; 0.05), and their interaction (p &lt; 0.01). The changes to arch height and area were significantly dependent on the muscle combinations (p = 0.001 and p &lt; 0.001, respectively). The arch height and area increased under the loading combinations of APB, OPP, APB-sFPB, APB-OPP or APB-sFPB-OPP (p &lt; 0.05), but not under the combinations of sFPB (p = 0.893) or sFPB-OPP (p = 0.338). The carpal arch change under the APB-sFPB-OPP or APB-OPP loading was greater than that under the loading of APB-sFPB (p &lt; 0.001). This study demonstrated that thenar muscle forces exert biomechanical effects on the transverse carpal ligament to increase carpal arch height and area, and these increases were different for individual muscles and their combinations.