Removal torque of nail interlocking screws is related to screw proximity to the fracture and screw breakage.

Studies have shown that titanium implants can be challenging to explant due to the material's excellent biocompatibility and resulting osseointegration. Clinically, titanium alloy nail interlocking screws may require removal to dynamize a construct or revise the nail due to nonunion, infection, pain, or periprosthetic fracture. This study was designed to determine what variables influence the removal torque for titanium alloy interlocking screws. An intramedullary nail with four interlocking screws was used to stabilize a 1-cm segmental femoral defect in a canine model for 16 weeks. The animals were observed to be active following a several-day recovery after surgery. In six animals, the femora and implanted nail/screws were first tested to failure in torsion to simulate periprosthetic fracture of an implant after which the screws were then removed. In four additional animals, the screws were removed without mechanical testing. Both intraoperative insertional and extraction torques were recorded for all screws. Mechanical testing to failure broke 10/24 screws. On average, the intact screws required 70% of the insertional torque during removal while broken screws only required 16% of the insertional torque (p < 0.001). In addition, intact screws closer to the fracture required 2.8 times more removal torque than the outboard distal screw (p < 0.005). On average, the angle of rotation to peak torque was ∼80°. The peak axial load did not significantly correlate with the torque required to remove the screws. On average, the removal torque was lower than at the time of insertion, and less torque was required to remove broken screws and screws remote to the fracture. However, broken screws will require additional time to retrieve the remaining screw fragment. This study suggests that broken screws and screws in prematurely active patients will require less torque to remove.

Angle stable nails provide improved healing for a complex fracture model in the femur.

BACKGROUND: Conventional nails are being used for an expanding range of fractures from simple to more complex. Angle stable designs are a relatively new innovation; however, it is unknown if they will improve healing for complex fractures. QUESTIONS/PURPOSES: When comparing traditional and angle stable nails to treat complex open canine femur fractures, the current study addressed the following questions: do the two constructs differ in (1) radiographic evidence of bone union across the cortices; (2) stability as determined by toggle (torsional motion with little accompanying torque) and angular deformation; (3) biomechanical properties, including stiffness in bending, axial compression, and torsional loading, and construct failure properties in torsion; and (4) degree of bone tissue mineralization? METHODS: Ten hounds with a 1-cm femoral defect and periosteal stripping were treated with a reamed titanium angle stable or nonangle stable nail after the creation of a long soft tissue wound. Before the study, the animals were randomly assigned to receive one of the nails and to be evaluated with biomechanical testing or histology. After euthanasia at 16 weeks, all operative femora were assessed radiographically. Histological or biomechanical evaluation was conducted of the operative bones with nails left in situ compared with the nonoperative contralateral femora. RESULTS: Radiographic and gross inspection demonstrated hypertrophic nonunion in all 10 animals treated with the nonangle stable nail, whereas six of 10 animals treated with the angle stable nail bridged at least one cortex (p = 0.023). The angle stable nail construct demonstrated no toggle in nine of 10 animals, whereas all control femora exhibited toggle. The angle stable nail demonstrated less angular deformation and toggle (p ≤ 0.005) and increased compressive stiffness (p = 0.001) compared with the conventional nonangle stable nail. Histology demonstrated more nonmineralized tissue in the limbs treated with the conventional nail (p = 0.005). CONCLUSIONS: Angle stable nails that eliminate toggle lead to enhanced yet incomplete fracture healing in a complex canine fracture model. CLINICAL RELEVANCE: Care should be taken in tailoring the nail design features to the characteristics of the fracture and the patient.