An in vitro biomechanical investigation of an intramedullary nailing technique for repair of third metacarpal and metatarsal fractures in neonates and foals

Objective— To evaluate a dorsoproximal extra‐articular approach for insertion of 8.25‐mm, solid‐titanium, intramedullary (IM) interlocking nails into ostectomized foal third metacarpal (MC3) and third metatarsal (MT3) bones; to compare the monotonic mechanical properties of IM nail constructs with paired intact bones; and to determine the effects of age, body weight, fore‐ or hindlimb, and left or right limb on the mechanical testing variables. Animal or Sample Population— Twenty bone pairs (10 MC3, 10 MT3) collected from 10 foals of various weights and ages. Methods— One bone from each pair was randomly selected to be ostectomized and stabilized using an 8.25‐mm, solid‐titanium IM nail, and four 3.7‐mm titanium interlocking screws (construct). Constructs and contralateral intact bone specimens were tested in axial compression and palmaro‐/plantarodorsal (PD) 4‐point bending. Monotonic mechanical properties were compared between intact specimens and constructs with an ANOVA; significance was set at P < .05. Results— Nail insertion caused bone failure in 6 MC3 and 2 MT3. In general, mean mechanical testing values indicated that intact specimens were significantly stronger and stiffer than constructs for all age and weight ranges when tested in compression and PD 4‐point bending (P < .05). Bone strength and stiffness of intact specimens tested in compression and bending tended to increase linearly with age and weight. Conclusions— IM interlocking nail fixation of gap‐ostectomized MC3 and MT3 with 8.25‐mm IM nails and 3.7‐mm interlocking screws did not achieve sufficient strength or stiffness to be recommended as the sole means of repair for comminuted MC3 and MT3 fractures in young foals. Clinical Relevance— IM interlocking nail fixation of foal cannon bone fractures may be useful to decrease soft‐tissue disruption at the fracture site; however, there is a risk of bone failure associated with extra‐articular insertion. This method should be combined with other forms of external coaptation for added stability in axial compression and PD bending.