Original Research

Effect of Plate in Close Proximity to Empty External-Fixation Pin Site on Long-Bone Torsional Strength

Am J Orthop. 2017 March;46(2):E108-E111

References

1. Brooks DB, Burstein AH, Frankel VH. The biomechanics of torsional fractures. The stress concentration effect of a drill hole. J Bone Joint Surg Am. 1970;52(3):507-514.

2. Edgerton BC, An KN, Morrey BF. Torsional strength reduction due to cortical defects in bone. J Orthop Res. 1990;8(6):851-855.

3. Gardner MP, Chong AC, Pollock AG, Wooley PH. Mechanical evaluation of large-size fourth-generation composite femur and tibia models. Ann Biomed Eng. 2010;38(3):613-620.

4. Wysocki RW, Sheinkop MB, Virkus WW, Della Valle CJ. Femoral fracture through a previous pin site after computer-assisted total knee arthroplasty. J Arthroplasty. 2008;23(3):462-465.

5. Burstein AH, Currey J, Frankel VH, Heiple KG, Lunseth P, Vessely JC. Bone strength. The effect of screw holes. J Bone Joint Surg Am. 1972;54(6):1143-1156.

6. Clark CR, Morgan C, Sonstegard DA, Matthews LS. The effect of biopsy-hole shape and size on bone strength. J Bone Joint Surg Am. 1977;59(2):213-217.

7. Evans PE, Thomas WG. Tibial fracture through a traction-pin site. A report of two cases. J Bone Joint Surg Am. 1984;66(9):1475-1476.

8. Stoffel K, Dieter U, Stachowiak G, Gächter A, Kuster MS. Biomechanical testing of the LCP—how can stability in locked internal fixators be controlled? Injury. 2003;34(suppl 2):B11-B19.

9. Klaue K, Fengels I, Perren SM. Long-term effects of plate osteosynthesis: comparison of four different plates. Injury. 2000;31(suppl 2):B51-B62.

10. Uhthoff HK, Poitras P, Backman DS. Internal plate fixation of fractures: short history and recent developments. J Orthop Sci. 2006;11(2):118-126.

11. Takemoto RC, Sugi MT, Kummer F, Koval KJ, Egol KA. The effects of locked and unlocked neutralization plates on load bearing of fractures fixed with a lag screw. J Orthop Trauma. 2012;26(9):519-522.

12. Wagner M. General principles for the clinical use of the LCP. Injury. 2003;34(suppl 2):B31-B42.

13. Strauss EJ, Schwarzkopf R, Kummer F, Egol KA. The current status of locked plating: the good, the bad, and the ugly. J Orthop Trauma. 2008;22(7):479-486.

14. Elfar J, Menorca RM, Reed JD, Stanbury S. Composite bone models in orthopaedic surgery research and education. J Am Acad Orthop Surg. 2014;22(2):111-120.

Discussion

Many tibia fractures require provisional stabilization with an external fixator that spans the knee, because of the high-energy nature of the injury or other, higher-priority polytrauma concerns. When the patient or injury is suitable for definitive fixation, the external fixator typically is removed in favor of internal fixation with a plate and screws. Depending on the nature and location of the fracture and the subsequent plate, the empty cortical pin-site defects, often lying at varying distances from the distal end of the plate, can potentially serve as stress risers for fracture.⁴

Other studies have evaluated long-bone cortical defects biomechanically^1,2,4 and clinically,^5-7 and multiple studies have been conducted on the effects of plates on long-bone strength for fracture stabilization.^8-13 The present study evaluated the torsional strength of long bones in the presence of a bicortical defect and the proximity of the defect to a plate. There were no differences in stiffness or failure load between any of the groups of plated and unplated fourth-generation composite tibias tested to failure in torsion with varying distal bicortical defects. Hypothetically, one would expect the torsional stiffness of these specimens to increase with the mere addition of a metallic diaphyseal plate. However, this study demonstrated that the addition of a plate did not affect the torsional stiffness or strength of the tibias. Clinically, it is common practice to place external fixator pins as far as possible outside the planned incision site for definitive fracture fixation. Thus, we also hypothesized that the presence of a bicortical pin-site defect and its proximity to the plate would alter the torsional strength of the tibia specimens, and that the distal pin-site defect’s location farthest from the plate would exhibit greater strength, but this did not occur. Although other studies have shown that the presence of bicortical defects decreases the strength of long bones, we were unable to quantify this decrease because the 2 intact groups of composites, plated and unplated, survived failure testing.

This study had several limitations, first being the use of composite tibias as opposed to human cadaver bone. Although fourth-generation composite bone models have been validated as a suitable and accurate biomechanical substitute for cadaver specimens,¹⁴ anatomical variations in cadaver tibias may transfer forces differently through plates, screws, and distal pin sites. In order to test plated specimens against the unplated controls, we did not simulate a mid-shaft fracture in any of the tibias. The pin-site defects were intended to reflect the mechanical effects of bicortical defects immediately after pin removal and in the absence of any degree of bone healing. Finally, this study focused on pin-site defects that were distal to a midshaft plate and that may not represent the effects of bicortical pin-site defects proximal to the plate.

Given the results of this biomechanical study in composite tibias, varying the proximity of a bicortical defect to a plate does not affect the torsional stiffness or torsional failure strength of the bone. Placement of an intended bicortical defect should be based on considerations other than the potential for creating a distal stress riser after definitive fracture management.

Effect of Plate in Close Proximity to Empty External-Fixation Pin Site on Long-Bone Torsional Strength

References

Discussion

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