Date Published: January 13, 2017
Publisher: Public Library of Science
Author(s): Jesse Chieh-Szu Yang, Hsin-Chang Chen, Yu-Shu Lai, Cheng-Kung Cheng, Jose Manuel Garcia Aznar.
Fixation with a dynamic hip screw (DHS) is one of the most common methods for stabilizing intertrochanteric fractures, except for unstable and reverse oblique fracture types. However, failure is often observed in osteoporotic patients whereby the lag screw effectively ‘cuts out’ through the weak bone. Novel anti-migration blades have been developed to be used in combination with a lag screw (‘Blade Screw’) to improve the fixation strength in osteoporotic intertrochanteric fractures. An in-vitro biomechanical study and a retrospective clinical study were performed to evaluate lag screw migration when using the novel Blade Screw and a traditional threaded DHS. The biomechanical study showed both the Blade Screw and DHS displayed excessive migration (≥10 mm) before reaching 20,000 loading cycles in mild osteoporotic bone, but overall migration of the Blade Screw was significantly less (p ≤ 0.03). Among the patients implanted with a Blade Screw in the clinical study, there was no significant variation in screw migration at 3-months follow-up (P = 0.12). However, the patient’s implanted with a DHS did display significantly greater migration (P<0.001) than those implanted with the Blade Screw. In conclusion, the Blade Screw stabilizes the bone fragments during dynamic loading so as to provide significantly greater resistance to screw migration in patients with mild osteoporosis.
Intertrochanteric fractures are common injuries in the elderly and are often a consequence of sudden impact to an osteoporotic hip, such as that experienced during a fall . Dynamic hip screw (DHS) is one of the primary choices for the fixation of intertrochanteric fractures (AO 31-A1). However, there is also an inherently failure risk with the use of a DHS, possibly down to the lag screw design. Madsen et al.  showed that 9% of patients receiving a DHS had a secondary fracture dislocation within a six month follow-up period, leading to complications such as varus malunion, lag screw cut-out or excessive lag screw sliding with medialization of the distal fracture fragment. In addition, postoperative fracture instability with secondary complications is frequently reported in severely osteoporotic patients [1–5]. In a sample of 178 intertrochanteric fractures treated with a DHS with a minimum of one year follow-up, Kim et al.  reported a complication rate in osteoporotic bone of greater than 50%. Poor bone quality is a major risk factor for intertrochanteric fractures, which could lead to an increased failure rate for implants or collapse and lag screw cut-out from the superior aspect in older patients [3,4].
This study compared lag screw migration between two lag screw designs, the Blade Screw and a traditional DHS, implanted in osteoporotic bone models. In the mild osteoporotic sawbone models, the Blade Screw group withstood a greater number of loading cycles before reaching the point of excessive migration of the implant (≥10 mm). This indicates a greater resistance to implant migration through the bone. Kouvidis et al. used similar testing protocols to investigate the number of loading cycles to implant failure in single and dual lag screw systems and showed that the average number of cycles leading to cut-out was 6,638 and 10,054, respectively . In the current study, the amount of loading cycles withstood by the traditional DHS and Blade Screw approximate those of single and dual lag screw systems, signifying the superior cut-out resistance of the Blade Screw. In addition, indicators of lag screw migration (DFH, DDLS, VC, RN) with the Blade Screw were all significantly less than with the traditional DHS (Table 1). This indicates the greater fixation strength of the Blade Screw construct under multidirectional dynamic loading. In addition, the low VC and RN of the Blade Screw also demonstrate its superior resistance to rotation, which is particularly important when the fracture line is perpendicular to the femoral neck. Rotation of a lag screw in this type of fracture would destabilize the femoral head fragments and result in varus collapse under multi-directional loading .
Compared to the traditional DHS, the Blade Screw provides significantly greater resistance to lag screw migration and cut-out in low density bones. The findings of this study show that the Blade Screw design has the advantage of improving the postoperative fixation in osteoporotic bones by securing the fracture fragments during dynamic loading.