Reconstruction after multilevel decompression of the cervical spine, especially in the weakened osteoporotic, neoplastic or infectious spine often requires circumferential stabilization and fusion. To avoid the additional posterior surgery in these cases while increasing rigidity of anterior-only screw-plate constructs, the authors introduce the concept of anterior transpedicular screw (ATPS) fixation. We demonstrated its morphological feasibility as well as its indications in a previous study in Part I of our project. Consequently, the objectives of the current study were to assess the ex vivo accuracy of placing ATPS into the cervical vertebra as well as the biomechanical performance of ATPS in comparison to traditional vertebral body screws (VBS) in terms of pull-out strength (POS). Twenty-three ATPS were inserted alternately to two screws into the pedicles and vertebral bodies, respectively, of six cadaveric specimens from C3-T1. For insertion of ATPS, a manual fluoroscopically assisted technique was used. Preand post insertional CT-scans were used to assess accuracy of ATPS insertion in the axial and sagittal planes. A newly designed grading system and accuracy score were used to delineate accuracy of ATPS insertion. Following insertion of screws, 23 ATPS and 22 VBS were subjected to pull-out testing (POT). The bone mineral density (BMD) of each specimen was assessed prior to POT. Statistical analysis showed that the incidence of correctly placed screws and non-critical pedicles breaches in axial plane was 78.3%, and 95.7% in sagittal plane. Hence, according to our definition of ''critical'' pedicle breach that exposes neurovascular structures at risk, 21.7% (n = 5) of all ATPS inserted showed a critical pedicle breach in axial plane. Notably, no critical pedicle perforation occurred at the C6 to T1 levels. Pull-out testing of ATPS and VBS revealed that pull-out ( ) 17:539-555 DOI 10.1007 resistance of ATPS was 2.5-fold that of VBS. Mean POS of 23 ATPS with a mean BMD of 0.566 g/cm 2 and a mean osseus screw purchase of 27.2 mm was 467.8 N. In comparison, POS of 22 VBS screws with a mean BMD of 0.533 g/cm 2 and a mean osseus screw purchase of 16.0 mm was 181.6 N. The difference in ultimate pull-out strength between the ATPS and VBS group was significant (p \ 0.000001). Also, accuracy of ATPS placement in axial plane was shown to be significantly correlated with POS. In contrast, there was no correlation between screw-length, BMD, or level of insertion and the POS of ATPS or VBS. The study demonstrated that the use of ATPS might be a new technique worthy of further investigation. The use of ATPS shows the potential to increase construct rigidity in terms of screw-plate pull-out resistance. It might diminish construct failures during anterior-only reconstructions of the highly unstable decompressed cervical spine.123 Eur Spine J
Stirrups aid the rider to stabilise their lower leg allowing it to be used effectively for communication and in maintaining their position in the saddle. Relatively few studies have investigated stirrup forces and to the best our knowledge no studies have reported stirrup forces in jumping. The aim of the present study was to measure stirrup forces in five showjumping horses ridden by the same professional rider. All horses were in regular training and competition jumping at least 30 cm higher than the fence used for the study. The fence chosen was a 70 cm upright with a pole at the top and a groundline. Right and left stirrup forces were measured using wireless load cells placed between the stirrup leathers and the stirrup. The signals were transmitted and digitised at 100 Hz and synchronised with video from a webcam using an inertial measurement unit. After warming-up, including over jumps, each horse attempted the jump three times from each rein in canter (3 horses left then right rein; 2 horses right then left rein). Mean peak total (sum of left and right) stirrup force for the approach (n=5 strides per horse per jump), take-off and landing phase of the jump was 1,034±110, 1,042±284 and 1,447±256 N (range 905 to 1,815 N), respectively (mean ± standard deviation). There was no significant difference between right or left mean peak stirrup force during approach or take-off, but mean peak force was consistently higher on the right stirrup during the early phase of landing on either the right or left rein (right: 827±320 N; left: 615±336 N; P<0.05). In conclusion, the mean total peak stirrup forces measured in the present study in the same rider jumping five different horses over a 70 cm single upright fence are similar to previous reports of peak stirrup forces in gallop and consistent with observations of asymmetric loading of the saddle and horses’ backs by riders.
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