Lag screw fixation of medial malleolar fractures: a biomechanical, radiographic, and clinical comparison of unicortical partially threaded lag screws and bicortical fully threaded lag screws.

OBJECTIVES:
Bicortical fully threaded (FT) lag screw fixation (lag by method) is a technique for medial malleolar fixation that may provide advantages to partially threaded (PT) cancellous lag screws (lag by screw design). A direct comparison of the biomechanical properties and the clinical outcomes of these 2 methods of medial malleolar lag screw fixation were undertaken. The null hypotheses were that unicortical PT lag screws and bicortical FT lag screws had similar biomechanical and clinical outcomes.

DESIGN:
Biomechanics--Human cadaver biomechanical investigation. Clinical-Retrospective cohort series with control. Settings-Level-1 trauma center.

PATIENTS:
One hundred forty-one consecutive patients with closed medial malleolar fractures (OTA 44) treated with lag screw fixation were identified from a prospective orthopaedic trauma database. Thirty-nine were lost to follow-up and 12 were treated with a single screw leaving 46 in the PT group and 46 in the FT group all treated with 2 screws for their medial malleolar fracture.

INTERVENTION:
Biomechanics--Human cadavers (n = 3) had bilateral oblique medial malleolar fractures (n = 6) created with an osteotome to simulate a typical medial malleolar fracture amenable to lag screw fixation. Fixation of each side was randomly allocated to either two 4.0-mm PT lag screws (length 45 mm) or two 3.5-mm FT threaded screws [length determined by depth gauge after near cortex overdrilled and far cortex (distal lateral tibia) drilled based on core diameter (2.7 mm)]. Clinical-Either 2 traditional PT (n = 46) or 2 FT screws (n = 46) were used for fixation of medial malleolus fractures.

MAIN OUTCOME MEASUREMENTS:
Biomechanics--Peak insertion torque generated during screw insertion. Clinical: Radiographic evidence of screw loosening, clinical non-union, and reoperation.

RESULTS:
Biomechanics--The FT lag screw group (n = 6 screws) showed an average maximum torque of 14.4 in-lbf (range 8.0-20.1, SD = 4.4) before screw stripping. This was over 3 times greater than that seen with the PT lag screws (n = 6) (average maximum torque generation = 4.0 in-lbf, range 2.5-6.6, SD = 1.4), P < 0.0002. Clinical-Radiographic screw loosening was evident in one of the 46 patients (2%) in the FT cohort and in 11 of the 46 patients (24%) in the PT cohort, P < 0.003. Two of the patients with screw loosening in the PT cohort required reoperation for removal of symptomatic hardware, whereas no patient from the LT screw group required removal. All patients in the LT cohort healed after the index procedure although 2 in the PT cohort had nonunions.

CONCLUSIONS:
Screws placed with the lag by method technique that engaged the distal lateral tibial cortex have superior biomechanical, radiographic, and clinical outcomes compared to traditional PT screws for the fixation of medial malleolar fractures.