Locked-in with OSSIS: Locking screws and fixed-angle stability
Stable fixation is one of the primary goals of orthopedic surgery. Along with good surgical technique and sound decision making, choosing the right implant is paramount to achieving this outcome. Maximal stability translates to a reduced risk of failure and improved patient quality of life.
To this end, OSSIS implants are made with the "one implant for life" philosophy. We overcome the limitations of standard implant options through the innovative use of 3D printing technology. Since 2007, OSSIS has been producing patient-specific titanium implants with highly osseointegrative porous structure designs. These implants have consistently provided superior results over traditional solutions, representing the new frontier in orthopedic implant design.
Further, OSSIS implants are set apart through their incorporation of locking screws for custom acetabular prosthesis. Having great utility in the fields of trauma and spine surgery, the locking screw mechanisms and their fixed-angle stability further add to the OSSIS guarantee of long-term durability.
Locking screws: An orthopedic innovation that’s here to stay
The year 1995 saw an increasing trend towards the use of locking plates. Pioneered by the Arbeitsgemeinschaft fur Osteosynthesefragen (AO) Foundation, orthopedic surgeons were introduced to the Less Invasive Stabilization System (LISS) and the locking compression plates (LCP). Surgeons have never looked back, and the locking plate remains an important part of the orthopedist’s armamentarium, particularly for osteopenic bone and complex trauma cases.
Locking screws and fixed-angle implants represent a change in treatment concept rather than a direct evolution of conventional plate and screw constructs. These implants behave more like external fixators in terms of their mechanics. For this reason, many in the orthopedic community refer to locking implants as “internal fixators”.
The AO locking screws have conical screw heads that lock within the threaded hole in the plates. They also incorporate fine-pitched threads designed for seamless advancement into bone. Because of its symmetrical design, the force exerted by the screw on the bone is equal in both pullout and advancement.
Whether used in plates or with other types of orthopedic implants, locking screws are rigidly fixed creating a screw-implant construct with high angular stability, torsional resistance, and shearing resistance. In this situation, the screw is imparted with a strong cantilever property that allows the fixation of the construct onto the bone. By and large, these are more stable and versatile implants compared to the conventional plates and screws of old.
Locking screws for poor bone quality
With an evolving treatment approach favoring minimally invasive surgeries, locking plate and screw constructs became an attractive option to achieve stable fixation whilst preserving soft tissue integrity. Previously, conventional plate and screw fixation methods relied on achieving sufficient bony purchase to achieve contact pressure and subsequent stability. This also meant conducting an extensive surgery, compromising bony union through the disruption of bone periosteum and blood supply.
For that reason, the use of conventional plates and screws in the setting of poor bone quality becomes heavily problematic. The literature is clear to point out how the pullout strength of standard screws is compromised in osteoporotic bone. Standard screws can have as much as 50% to 70% decrease in pullout strength in osteoporotic bone (1).
Standard screws cannot adequately produce enough counter torque to resist external forces. Further, the application of conventional implants necessitates compression of the bone underneath. This produces necrosis of the underlying bone and becomes counterintuitive in the setting of osteopenia or bone loss.
Custom-implant Prosthesis Fixation: Improving stability through locking screws
Challenging surgical scenarios such as revision hip arthroplasty or pelvic reconstruction for periacetabular metastatic disease demands achieving rigid fixation in a setting of poor bone quality. In this situation, a surgeon must make effective use of an implant that ensures bony ingrowth to achieve surgical success.
OSSIS implants were made with these scenarios in mind. With a highly-osseointegrative porous structure and a customized design to address specific anatomical defects, OSSIS provides a robust-performing implant meant to provide superior stability. Coupled with a of the variety of locking screws to choose from in the OSSIS screw family, OSSIS implants are locked-in for long-term fixation. The trajectory and type of the locking screws are specifically designed for each OSSIS implant. We ensure the screws are diverging and go into higher quality bone, adding to the pull-out strength and optimising stability, whilst maintaining surgical accessibility.
Research supports the use of fixed-angle screws for acetabular prosthesis fixation. Hugate and colleagues compared fixed-angle screws versus standard screws in the fixation of an acetabular prosthesis in a cadaveric biomechanical study (2). They performed an analysis of the fixation strength of acetabular cups with fixed-angle screw fixation and to compare it with that of standard screw fixation.
Sequential pullout of conventional screws.
Image source: P. Croniera, G. Pietub, C. Dujardinc, N. Bigorrea, F. Ducelliera, R. Gerardd. WORKSHOPS OF THE SOTO (2009 RENNES). The concept of locking plates. Orthopaedics & Traumatology: Surgery & Research 96S, S17—S36, 2010.
En Bloc pullout of an interlocking screw system.
The study included 8 cadaveric pelves of which fixed-angle lock screws were used as the means of fixation for the prostheses in all left acetabulum specimens while standard screws were used for all right acetabulum specimens. Locking screw heads engaged threaded receptacles in the acetabular prosthesis directly – thereby creating a fixed-angle construct. They performed cyclic and load-to-failure testing to compare the stability of the two screw types.
The results appear to favor fixed-angle screws as the more rigid construct. The yield moment was significantly higher for those prostheses fixed with fixed-angle screws as compared to standard screws (p<0.02). This difference between the two groups represented a mean increase in yield moment of 40% in fixed-angle screw constructs versus those using standard screws (p=0.04). Furthermore, the fixed angle screw specimens demonstrated less initial displacement (0.82 vs 2.2) when compared with the standard screw groups.
Conclusion
Through its incorporation of locking screws and fixed-angle stability, customized OSSIS implants provide reliable and superior implant fixation for the treatment of challenging surgical scenarios with poor bone quality. OSSIS implants are designed for long-term stability and durability, highlighting the OSSIS “one implant for life” philosophy.
References
Soshi, S., Shiba, R., Kondo, H. et al. An experimental study on transpecidcular screw fixation in relation to osteoporosis of the lumbar spine. Spine 1991; 16:1335.
Hugate RR, Dickey ID, Chen Q, Wood CM, Sim FH, Rock MG. Fixed-angle screws vs standard screws in acetabular prosthesis fixation: a cadaveric biomechanical study. J Arthroplasty. 2009 Aug;24(5):806–14.