Lumbar vertebral adaptation and bone stress injury in adolescent cricket fast bowlers

Cricket fast bowling generates substantial tri-planar trunk movement and ground reaction forces (GRF), and senior fast bowlers (FB) show marked lumbar spine bone mineral adaptation, specifically on the contralateral (CL) side of the vertebrae to their bowling arm, along with a high prevelance of lumbar bone stress injuries. It is unknown how adaptation develops in FB nor how it associates with loading characteristics. This thesis aimed to evaluate the adaptation of lumbar vertebrae in adolescent FB and how it differs according to skeletal and chronological age, bowling technique, or cricket and bone loading history, as well as potential risk factors for lumbar bone stress injury.

41 adolescent male FB aged 14-17 years were compared to 11 field sport controls, 41 age matched typical controls (TC), and 66 senior FB. All participants received an AP lumbar spine DXA scan and lumbar MRI scan, and 27 FB were followed up for a median of 2.22 years. FB underwent 3D motion analysis of their bowling technique, musculoskeletal measurement, and skeletal maturation assessment annually, as well as longitudinal recording of bowling workload, physical activity and injury monitoring.

FB had substantially higher lumbar bone mineral, vertebral area, and width than TC. Bone mineral density Z-score of TC seemed more typical compared to the reference population than that of FB who were nearly 1 SD higher (mean ± SD L1-L4 Z-score of +0.19 ± 0.82 and +0.84 ± 1.19 respectively). Notably, FB had unique asymmetry within the lumbar spine with up to 8.66% higher bone mineral density on the CL side of L3, whereas in controls, bone mineral content was up to 7.08% greater on the ipsilateral side of L2. In FB, lumbar vertebral bone mineral and dimensions increased with age with significantly greater annual changes in younger FB compared to older. Peak GRF in Newtons and thoraco-lumbar maximum CL rotation predicted up to 44.6% variance in lumbar bone mineral. The incidence of prospective lumbar bone stress injury was 27.3 ± 18.6 injuries per 100 players per year, and chronological age was significantly greater in injured bowlers compared to uninjured.

Adaptation to fast bowling substantially increases with age, particularly on the CL side. Increased bone strain during the bowling action, resulting from multidirectional forces, could increase adaptation at specific lumbar sites. Adolescent fast bowlers are at increased risk of lumbar bone stress injuries which may coincide with increases in bowling workload and intensity as fast bowlers step up playing levels whilst the lumbar spine is immature and less robust.