The effect of surgeon-controlled variables on construct stiffness in lateral locked plating of distal femoral fractures

Abstract

Background: Nonunion following treatment of supracondylar femur fractures with lateral locked plates (LLP) has been reported to be as high as 21 %. Implant related and surgeon-controlled variables have been postulated to contribute to nonunion by modulating fracture-fixation construct stiffness. The purpose of this study is to evaluate the effect of surgeon-controlled factors on stiffness when treating supracondylar femur fractures with LLPs: 1. Does plate length affect construct stiffness given the same plate material, fracture working length and type of screws? 2. Does screw type (bicortical locking versus bicortical nonlocking or unicortical locking) and number of screws affect construct stiffness given the same material, fracture working length, and plate length? 3. Does fracture working length affect construct stiffness given the same plate material, length and type of screws? 4. Does plate material (titanium versus stainless steel) affect construct stiffness given the same fracture working length, plate length, type and number of screws?

Methods: Mechanical study of simulated supracondylar femur fractures treated with LLPs of varying lengths, screw types, fractureworking lenghts, and plate/screw material. Overall construct stiffness was evaluated using an Instron hydraulic testing apparatus.

Results: Stiffness was 15 % higher comparing 13-hole to the 5-hole plates (995 N/mm849N vs. /mm, p = 0.003). The use of bicortical nonlocking screws decreased overall construct stiffness by 18 % compared to bicortical locking screws (808 N/mm vs. 995 N/mm, p = 0.0001). The type of screw (unicortical locking vs. bicortical locking) and the number of screws in the diaphysis (3 vs. 10) did not appear to significantly influence construct stiffness (p = 0.76, p = 0.24). Similarly, fracture working length (5.4 cm vs. 9.4 cm, p = 0.24), and implant type (titanium vs. stainless steel, p = 0.12) did also not appear to effect stiffness.

Discussion: Using shorter plates and using bicortical nonlocking screws (vs. bicortical locking screws) reduced overall construct stiffness. Using more screws, using unicortical locking screws, increasing fracture working length and varying plate material (titanium vs. stainless steel) does not appear to significantly alter construct stiffness. Surgeons can adjust plate length and screw types to affect overall fracture-fixation construct stiffness; however, the optimal stiffness to promote healing remains unknown.

Keywords: Biomechanics; Distal Femur; Fracture; LISS; Lateral Locked Plating; Nonunion; Supracondylar Femur.

Fig. 1

A diagram representing the different fracture-fixation constructs tested with the comparisons made. In each case the bold/underlined variable is the change made from the standard testing model. All constructs were tested in both titanium and stainless steel to compare the effect of plate and screw material

Fig. 2

An illustration of the various constructs tested indicating plate length, screw type and pattern, fracture working length and plate material

Fig. 3

The mechanical testing apparatus used for the study

Fig. 4

Comparisons of overall fracture-fixation construct stiffness. a Condition 1: 5-hole plate compared to 13-hole plate. b Condition 2: bicortical locking screws compared to bicortical nonlocking screws. c Condition 3: bicortical locking screws compared to unicortical locking screws. d Condition 4: 3 screws compared to 11 screws filling the proximal portion of the plate. e Condition 5: short fracture working length compared in longer fracture working length. f Condition 6: stainless steel plate and screw constructs compared to titanium plate and screw constructs