Midshaft Clavicle

Midshaft fractures account for 80% of clavicle fractures. Traditionally, even displaced fractures managed with conservative treatments based on the satisfactory results of several previous studies. However, recent studies have reported unsatisfactory results of conservative treatments. A more aggressive attitude with surgical treatment had been suggested. Surgical implants of midshaft clavicle fixation can summarize into two types; plates and intramedullary fixation.

To reduce the incidence of wound infection and fracture nonunion; a closed or minimally invasive method using intramedullary fixation with cannulated screws had proposed. However, there are no mechanical studies on the use of cannulated screws in the fixation of midshaft clavicle fractures. The hollow structure of a cannulated screw may weaken the mechanical strength of the implant.

The intramedullary devices

The larger diameter and shorter intramedullary purchase length of the cannulated screw; compared with other intramedullary devices, may a drawback. This study evaluated the construct bending stiffness of cannulated screws used to repair a simulated midshaft fracture of synthetic clavicles under a cantilever bending test. It was hypothesized that the mechanical strength of a cannulated screw would be at least equal to that provided by a Knowles pin or a reconstruction plate.

After transverse osteotomy over the midpoint for fracture simulation; eighteen synthetic clavicles assigned to 3 groups and fixed with reconstruction plate, Knowles pin or cannulated screw. Purchase length was defined as the engaged length of the intramedullary portion of the two intramedullary devices Stiffness, yield load and maximum load of the cantilever bending test calculated of each tested synthetic bones.

The Knowles pin group had a significantly longer average intramedullary purchase length compared with that of the cannulated screw group. The construct stiffness in the reconstruction plate group (5.6 ± 0.9 N/mm); was higher than that of the intramedullary devices; the Knowles pin group (3.1 ± 0.6 N/mm); provided a greater construct stiffness than did the cannulated screw group (1.7 ± 0.4 N/mm) (p = 0.007). The cannulated screw group had the lowest yield and maximum load compared with the reconstruction plate and Knowles pin groups.

Implant-bone interface

Both the reconstruction plate and Knowles pin failed at the implant-bone interface. However, the cannulated screw group failed at the osteotomy site with broken implants. This study suggests that fixation of midshaft clavicle fractures with cannulated screws may lead to early failure due to inadequate mechanical strength. Ideal intramedullary clavicle devices should supply adequate intramedullary purchase lengths and mechanical strength.
Therefore this study investigated the mechanical effects of three types of fixation implant (reconstruction plate, Knowles pin, and cannulated screw); so for a simulated midshaft transverse osteotomy using synthetic clavicles. The results showed that the reconstruction plate had the greatest bending stiffness, yield, and maximum load; by contrast, the cannulated screw group had the lowest values for all three measures.