OTA 1997 Posters - Tibia Fractures

The Mechanical and Clinical Efficacy of Blocking Screws in Stabilizing Tibia Fractures with Short Proximal or Distal Fragments using Small Diameter Intramedullary Nails

Christian Krettek, MD, Theodore Miclau, MD, Jürgen Mannß, Christine Stephan, Urs Möhlmann

Hannover Medical School, Hannover, Germany

Introduction: In contrast to the intramedullary fixation of diaphyseal tibia fractures, the nailing of metaphyseal fractures with short proximal or distal fragments is associated with an increase in frontal and sagittal plane malalignment. The incidence of malalignment is due, in part, to the instability which results from the great difference between the diameters of the implant and the medullary cavity in the metaphysis. As the nail does not contact cortical bone, it and may translate along the interlocking screws because of the play between the screws and nail holes. One solutions to this problem is the use of blocking screws which functionally decrease the width of the metaphyseal medullary cavity, block transverse nail translation, and, theoretically, increase the stiffness and strength of the bone-implant-construct. The purpose of this study was to evaluate the mechanical stability and clinical efficacy of blocking screws in supplementing small diameter intramedullary nail fixation of high proximal and low distal tibial fractures.

Materials and Methods: Mechanical study: Ten intact fresh human cadaveric tibiae were dissected from the soft tissues, stored at 25° Celsius, and later thawed at room temperature for 24 hours prior to being used. The tibiae were sectioned to provide 10 distal segments measuring 70mm and 10 distal segments measuring 90mm. In the distal segments, commercially available stainless steel solid 8 millimeter tibial nails (Synthes) were advanced to 8mm of the ankle joint. Two transverse and 1 anterior-posterior 3.2/3.9mm locking screws were drilled and inserted using a custom made jig. The same jig also was used for the placement of a medial and a lateral blocking screw (BS) in anterior-posterior direction, 9mm above the upper transverse locking screw. In the proximal segments, the upper end of the nail was kept at the level of the anterior-superior edge of the entry point. One static and 1 dynamic interlocking screw measuring 3.2/3.9mm diameters were placed in medial-lateral direction with the use of the insertion handle. A jig was also used for the placement of medial and a lateral blocking screw (BS) in anterior-posterior direction, 9mm below the lower transverse locking screw and 16mm proximal to the lower end of the segment. The bone-implant-construct (BIC) was embedded into bone cement in a cast frame. The BIC was fixed in a materials testing machine (MTM). The distal BICs were loaded in the medial-lateral direction 185mm proximal to the nail tip with loads from -150 N to + 150 N. The proximal BICs were loaded similarly 185mm from the proximal nail end. Force-displacement curves were recorded before (control) and after the insertion of the BS.

Clinical study: Between 7/93 and 7/96, 22 tibia fractures (11 proximal and 11 distal third; 5 A-, 9 B- and 7 C-types; 10 closed, 11 open) were stabilized with a locked small diameter unreamed tibial nail. Blocking screws were placed during the initial nailing (n=15) or in a subsequent procedure (n=6). Injury patterns, treatment, and pre- and postoperative x-rays were obtained and analyzed. One patient was lost for follow-up. Efficacy of BS was analyzed using a 3 parameter score (varus-valgus alignment 0-3 points, fracture healing 0-3 points, absence of a cast 0-1 point).

Results: Mechanical study: In distal BICs, the addition of blocking screws decreased the average deformation of the BIC 57% from 9.5° ± 2.1° in the control group to 4.0° ± 1.1° (mean ± SD) in the BS group (p<0.0001). In proximal BICS, the addition of blocking screws decreased the average deformation of the BIC 25% from 8.9° ± 1.0° in the control group to 6.8° ± 1.2° in the BS group (mean ± SD) (p<0.0001).

Clinical study: At follow up (average 17 months) all cases had healed. Correction of axial malalignment was within a varus-valgus 3° in 17 cases and within a varus-valgus of 4 to 5° in 4 cases. However, in 4 cases, a cast was applied for additional stabilization. Efficacy of BS was rated as excellent in 14 cases (67%), good in 4 cases (19%), as poor in 3 cases (14%) and none of them as failure.

Conclusions: Based on the results of the mechanical study, the authors conclude that blocking screws can increase the primary stability of distal and proximal metaphyseal fractures. From the clinical study, the authors conclude that blocking screws can be effective for selected cases of proximal or distal fifth tibia fractures which exhibit malalignment and/or instability after nailing.