The Quest for Flexible Fixation with Locking Plates

By Michael Bottlang, PhD, Director, Legacy Biometrics Laboratory

Tuesday, January 4th, 2011

A 2004 editorial entitled ‘‘When Evolution Begets Revolution’’ described locking plates as the next great advance in orthopaedic traumatology that was adopted at an unprecedented speed [1]. The editorial concluded with the prudent prediction that ‘‘this wave of enthusiasm will surely be followed with an analysis of the inherent problems, followed by a truer understanding of the role of these implants.” Today, locking plates are recognized for the superior fixation strength of fixed-angle locking screws, particularly for metaphyseal fixation in osteoporotic bone. They furthermore support biological fixation, allowing subcutaneous plating while preserving periosteal perfusion. Hence, they satisfy two out of three principal aspects of fracture fixation, being stable fixation, preservation of biology, and promotion of fracture healing.

The latter aspect of fracture healing is increasingly being recognized as an inherent problem of the current generation of locking plates. If locking plates do not provide a mechanical environment that promotes fracture healing, they become prone to losing the race between healing and fixation failure, leading to late implant breakage and loss of fixation. There is growing evidence from clinical and animal studies that the inherent stiffness of a locked plating construct can suppresses fracture motion to a level that is insufficient to promote fracture healing by callus formation [2-4].

With hindsight, locked bridge plating constructs pose an apparent conundrum: They provide inherently rigid stabilization, yet they should facilitate secondary bone healing that relies on flexible fixation to stimulate callus formation. To resolve this conundrum, we developed a modified locked plating concept, termed Far Cortical Locking (FCL) that enables flexible fixation with locking plates [4,5]. FCL reduces the stiffness of a locked plating construct by means of FCL screws that are fixed in the plate and in the far cortex while retaining a controlled motion envelope in the near cortex of a diaphysis. FCL screws have a flexible shaft with a reduced diameter that can elastically deflect within the near cortex motion envelope. The motion envelope is controlled by the diameter of a collar segment adjacent to the FCL screw head. FCL constructs therefore resemble a monolateral external fixator, the bar of which has been applied close to the bone surface and the pins of which are secured in the far cortex rather than in the near cortex.

A biomechanical study has shown that FCL screws reduce the stiffness of locked plating construct by over 80% while retaining comparable strength [5]. An in vivo study has furthermore shown that FCL constructs reliably yielded bridging of all cortices, causing healed fractures to be 156% stronger than control fractures stabilized with standard locked plating constructs [4]. Most interestingly, standard locked constructs suppressed fracture healing at the cortex under the plate where fracture motion is minimal. A clinical study is currently being conducted to document the effect of FCL screws on healing of supracondylar femur fractures.

The facts that controlled interfragmentary motion can promote fracture healing while absence of motion will suppress callus formation have long been recognized in the external fixator literature, particularly by the landmark studies of Goodship and Kenwright [6] and Claes [7]. As such, the evolution of locked plating towards more flexible fixation is both novel and conservative. If clinical results should support the prior finding on improved healing with FCL, they will likely inspire a variety of implant solutions aimed at providing flexible fixation with locking plates. These solutions will be key for the quest on flexible fixation with locking plates. However, solutions should be supported by evidence on their ability to promote fracture healing while ensuring that flexible fixation is not gained on cost of fixation strength. Such next generation of flexible locking plates will resemble true internal fixators that replicate the biomechanical behavior of external fixators by allowing adequate interfragmentary motion to promote the natural fracture healing cascade via callus formation. It is the hope of the author that this evolution will in turn resolve the misnomer “secondary” bone healing by recognizing the prime importance of this natural healing cascade for the vast majority of fractures.

Dr. Bottlang is the Director of the Legacy Biomechanics Laboratory in Portland, OR, USA. His research is focused on orthopaedic trauma and fracture care. His line of research on FCL was funded by the NIH and has received the Award of Excellence at the 2010 meeting of the American Association of Orthopaedic Surgeons. He holds several patents and has contributed to the development of several devices, including Zimmer “MotionLoc” FCL screws for which he receives royalties.

Note:  Listings of the MotionLoc FCL screws and the NCB Polyaxial Plate can be found on OrthopaedicLIST.com and x-ray examples of plate and screw fixation of fractures can be seen in the OrthopaedicLIST.com Implant Identification section.

[1] Sanders R. When evolution begets revolution. J Orthop Trauma. 2004;18:481-482.

[2] Henderson CE, Bottlang M, Marsh JL, Fitzpatrick DC, Madey SM. Does locked plating of periprosthetic supracondylar femur fractures promote bone healing by callus formation? Iowa Orthop J. 2008;28:73-6.

[3] Lujan TJ, Henderson CE, Madey SM, Fitzpatrick DC, Marsh JL, Bottlang M. Locked plating of distal femur fractures leads to inconsistent and asymmetric callus formation. J Orthop Trauma. 2010;24-3:156-62.

[4]  Bottlang M, Lesser M, Koerber J, Doornink J, von Rechenberg B, Augat P, Fitzpatrick DC, Madey SM, Marsh JL. Far cortical locking can improve healing of fractures stabilized with locking plates. The Journal of bone and joint surgery. 2010;92:1652-1660.

[5] Bottlang M, Doornink J, Fitzpatrick DC, Madey SM. Far Cortical Locking can reduce the stiffness of locked plating constructs while retaining construct strength. J Bone and Joint Surg. 2009; 91(8):1985-1994.

[6] Goodship AE, Kenwright J. The influence of induced micromovement upon the healing of experimental tibial fractures. J Bone Joint Surg Br. 1985;67-4:650-5.

[7] Claes LE, Wilke HJ, Augat P, Rubenacker S, Margevicius KJ. Effect of dynamization on gap healing of diaphyseal fractures under external fixation. Clin Biomech (Bristol, Avon) 1995;10-5:227-34.

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