Biomechanical Analysis Of The Pars Interarticularis Fracture In Athletes

Abstract

Pars interarticularis fracture (PIF) means that the bone between the superior and inferior articular is disconnected. For adults, the incidence is about 5%. Especially in L4 and L5 their incidence were seperately 85% and 15%. But among sportsmen the incidence is much higher, about 20.7%. The investigation among the athletes in 26 events showed that the volleyball player's incidence was the highest, about 50%. In other events it was: acrobatic gymnastics 47%. high hump 29%, javelin, pole vault and triple jump 27 %. In other events they were all under 20%. But in weightlifters it is only 8%. By analysing the events having higher incidence, it can be found that PIF often happens in the athletes needing more extending exercise. So that two experiments were done - spine specimen experiment and 3 dimensional photoelastic experiment, to analyse the PIF. The experiment of spine specimen was used to study and compare the forces born by the pars interarticularis in erect and extended spine. A special transducer was designed and horizontally put between L5 and 51. To analogue the different postures of the spine the load was applied in different methods. The experiments showed that in the erect spine, the shear force born by the pars interarticularis of the 5th lumber vertebra was 27% of the applying load, but in the extending spine it is 73%. These result are consistant with the above facts that the PIF often happens in the athletes who have to do more extending exercise and the incidence in weight lifter is the smallest. It was also acquired that the lumbar-sacrum angle is another important factor for the PIF. The bigger is the angle, the easier is the fracture. The photoelastic model of spine was made of epoxy resin. The ligments were substituted by the carbon fibre and the intervertebral discs by silicone rubber. The posterior joint caves were filled with a special material. Six slices, cut from the pars interarticularis of the erect and extended spine model, were analysed in the photoelastic meter. The result shows that the shearing stress distribution among the boundary of the pars in terarticularis in the extended spine is much higher than that in the erect spine. Its maximum shearing stress is about 2.7 times of the maximum one in other slices. It was verified again that PIF is much common in extended spine and its real reason is the shearing stress.