section epub:type=”chapter” role=”doc-chapter”> Janik C. Gasiorowski The basic principles of application of locking plates are the same in large animals as they are in the other species covered in this text. The benefits of locking implants include elimination of the need for plate/bone compression, greater construct stiffness, and biomechanical stability as compared to nonlocking implants, resistance to cyclic fatigue, and the general versatility offered by plate design [1]. These principles have been discussed in previous chapters but some bear repeating, as they are particularly relevant to internal fixation in large animals. Immediate restoration of limb function and return to weight bearing is of critical importance in large‐animal patients. Prolonged overbearing on the contralateral (support) limb leads to laminitis in the horse, laminitis or interdigital ligament breakdown in the cow, and angular limb deformity in the skeletally immature (developing) animal. The greater construct stiffness and stability achieved with the use of locking plates helps enable earlier return to function. Excessive rigidity of fixation has been implicated in delayed or nonunion complications with fracture healing [2]. This is rarely a problem in large‐animal patients because of the large cyclic loading imposed by their high body weights. It can be an issue in very young animals, but the superior healing characteristics of young bone usually overcome this disadvantage. Very few implants are designed specifically for use in large animals. As such, large animal surgeons often operate at the limits of the tolerances of the plates and screws available. Locking implants offer increased strength through advances in construct strength but also, in some cases, simply by offering more metal. The 5.5 locking compression plate (LCP) is thicker (6.0 mm) than Dynamic Hip Screw and Dynamic Condular Screw system (DHS/DCS) by Synthes (5.8 mm), and the finer threads of the locking head screws (LHS) offer a substantial increase in proportional core diameter as compared to cortex screws. More metal translates to an increase in area moment inertia and thus increases in implant rigidity and ultimate strength. The head of a locking screw cannot rotate or toggle in the plate, virtually eliminating pullout failure [1]. With conventional plating, the nonthreaded head of the cortex screw can toggle in the smooth plate hole, allowing a crowbar effect under bending load (i.e. the plate acts like a crowbar and the screw is like a nail being pried straight out of the substrate). Application of force in this manner potentiates pullout failure. In this scenario, only the bone that is between the threads of the cortex screw needs to fail for the screw to pull out. In a fixed‐angle construct, the threaded head cannot toggle in the threaded plate hole so there is no crowbar effect. An entire swath of bone would need to fail in compression, or the bone would have to fracture for failure of the plate/screw/bone construct to occur. This allows for use of much smaller threads on a locking screw. Smaller threads yield a bigger core shaft diameter and thus a stronger screw. For example: the core diameter of a 5.5 mm cortex screw is 4.0 mm, whereas the core diameter of a 5.0 mm locking screw is 4.4 mm (DePuy Synthes catalog 2017). The area moment inertia of the larger core diameter is 1.6 times greater, increasing substantially the bending and shear strength of the implant. The LCP has “cut‐outs” along its underside, which homogenizes its cross‐sectional area, normalizing the area moment of inertia for the length of the plate. This eliminates internal stress risers and allows smooth bending when contouring the plate. One end is rounded, allowing for juxta‐articular applications, while the other end is tapered, facilitating minimally invasive application. Another significant improvement increasing LCP versatility is the combi‐hole. The threaded potion of the hole is the root of its fixed‐angle advantages, but without the dynamic compression unit (DCU) portion of the hole, the plate would function only as a pure internal fixator. The DCU portion of the combi‐hole allows application in dynamic compression fashion, increasing exponentially the versatility and surgical value of the LCP. The DCU portion also allows angulation of cortex screws toward free fragments or away from neurovascular structures, joints, fracture gaps, and other implants. The combi‐hole of the 4.5 and 5.5 LCP can accommodate 4.5 and 5.5 mm cortex screws and 6.5 mm cancellous screws. The DCU portion of the hole allows 40° of longitudinal angulation and 7° of transverse angulation of a 4.5 mm cortex screw. It allows 25° of longitudinal angulation with a 5.5 mm cortex screw. Another improvement is the coaxial, or “stacked,” combi‐hole. This perfectly round hole is smooth at the top and threaded at the bottom, allowing use of a cortex or locking head screw. This hole configuration takes up less space within the plate than a regular combi‐ or DCU hole, which allows it to be located closer to the end of the plate, thus permitting closer approximation of the end screw to a joint. The increased rigidity and fixed‐angle stability of locking plates and screws make them a clearly superior choice over nonlocking implants for arthrodesis and fracture repair in large animals. Arthrodesis requires rigid fixation for bone fusion and for patient comfort. Construct rigidity results in less callus formation, decreasing the risk of impingement on the periarticular tissues and improving the cosmetic result. An LCP designed specifically for arthrodesis of the equine proximal interphalangeal joint is available (DePuy Synthes Vet, Paoli, PA). In this application, two abaxial (nonplate) transarticular screws engage the palmar/plantar processes of the middle phalanx and the plate is applied dorsally (Figure 6.1). Figure 6.1 Locking compression plate (LCP) geometry. The underside of the LCP has undercuts that limit the surface area of bone contact. These cutouts occur at the points in between holes, helping to homogenize cross‐sectional area for the length of the plate. (Source: Image credit: AO Foundation.) The specifics of plate application in this scenario warrant discussion because the technique differs from the original compression plating technique. The two transarticular screws are inserted and tightened compressing the palmar/plantar aspect of the joint. Countersinking is performed, taking care to remove bone proximally but not distally to prevent screw bending with asymmetrical contact between the screw head and bone when tightened. The plate is oriented with the stacked hole distally. The distal end is held firmly against the bone because the locking screw will not compress the plate to the bone surface in lag fashion. It can be held manually or with a push‐pull device in the middle hole. A locking screw is inserted and tightened. A cortex bone screw is placed in the central hole in the load position and tightened, compressing the dorsal aspect of the joint. A locking screw is placed in the proximal hole. The stacked hole and rounded end of the plate help prevent contact of the extensor process of the distal phalanx with the distal aspect of the plate at full extension of the distal interphalangeal joint. The LCP has been evaluated in vitro and demonstrated to be stiffer than an limited‐contact dynamic compression plate (LC‐DCP) construct [3]. The fact that less displacement was seen with the LCP construct during cyclic loading should translate to less callus formation in vivo. This is of particular importance in this location since horses are often expected to return to athletic performance after pastern arthrodesis. Excessive callus results in impingement on the soft tissues, most notably restriction of the long or common digital extensor tendon. In the author’s experience, a minimally invasive approach is preferable (when feasible) in cases of severe PIPJ arthritis. Preexisting cartilage destruction obviates arthrotomy and luxation. The PIP plate is introduced via subtendinous tunnel and all screws are inserted through stab incisions. Fetlock arthrodesis in the horse is a challenging endeavor. The most common reasons for failure are not directly related to the implants or surgical procedure. They include subluxation of the proximal interphalangeal joint, vascular trauma at the time of injury, infection, and contralateral limb laminitis [4]. Locked plating increases construct rigidity and reduces surgical time [5]. The LCP is applied in the same general manner as nonlocking plates. Since the plate is applied dorsally, on the bending surface of the construct, a tension band must be placed at the palmar/plantar aspect of the joint (Figure 6.2). A cortex screw is used near the joint space and is angled proximally into the dense bone of the condyles. Figure 6.2 Plate/screw PIPJ arthrodesis. A three‐hole 4.5 mm narrow (PIPJ‐specific) LCP was applied dorsally for arthrodesis of the proximal interphalangeal joint. Abaxial transarticular screws were placed in lag fashion to compress the palmar aspect of the joint. The specific advantage of the LCP in arthrodesis of the carpal joint(s) is engagement of the small carpal bones with a stronger screw at a fixed angle. It can be difficult or impossible to get more than one plate screw into the radial and ulnar carpal bones. The large core of the locking screw reduces the chances of implant failure. Additionally, the locking head eliminates the rotation and toggling that is possible with the smooth head of a cortex screws, reducing chances of implant loosening and increasing rigidity. Locked plating for pancarpal arthrodesis is initiated with a standard technique. The fracture(s) are reduced and the articular cartilage is removed. The craniomedial plate is positioned with the locking units of the central two combi‐holes directly over the radial and third carpal bones. A cortex screw is inserted through the plate in load position into the proximal aspect of the third metacarpal bone but not tightened. A second cortex screw is inserted in the load position into the distal aspect of the radius. These two screws are tightened to compress the carpal joints. The craniolateral plate is applied at this point with the same pattern of screw insertion. The craniolateral plate can be applied through the same incision. Some fracture configurations necessitate more lateral positioning of the craniolateral plate. Significant contouring is needed for adequate plate/bone contact in a nonlocking construct and a second incision is often required. Herein lies another advantage of the LCP, as perfect contouring is not required for stability and the second plate can be applied in a minimally invasive fashion. However, some contouring is still required – otherwise, skin closure will be very difficult. An additional cortex screw can be placed in load position on each side of the carpus in each plate. Locking screws are inserted into the remaining holes of both plates. Partial carpal arthrodesis is performed most commonly for comminuted fracture of small carpal bone(s) or collapse secondary to advanced osteoarthritis. As such, plates are applied to buttress the joints(s), preventing collapse. Locking plates are applied to the dorsomedial and dorsolateral aspects of the carpus (Figure 6.3). The dorsolateral plate is applied through the same incision or via minimally invasive approach. Locking screws are used when the small bones are intact or severely comminuted. If the small carpal bones have repairable damage (i.e. two‐piece fracture), cortex screws placed (through the plate) in lag fashion are used to stabilize the fracture. The advantages gained from stabilization of the fracture outweigh the advantages of a locking screw in this position. If the small bone fracture can be compressed with a screw outside of the plate, then a locking screw is used in this plate hole. Figure 6.3 Tension band wire used in metacarpophalangeal arthrodesis. Due to anatomical constraints the LCP must be applied dorsally, on the bending surface of the joint. Here, a palmar tension band wire was used to mitigate cyclic bending forces on the plate. Arthrodesis of the tarsometatarsal and distal intertarsal joints is performed for treatment of fracture, luxation, or osteoarthritis. A T‐plate (4.5 mm locking T‐plate, DePuy Synthes Vet, Paoli, PA) is available and well‐suited for this purpose [6]. The plate is applied dorsomedially (Figure 6.4) and three locking screws are inserted into the central tarsal bone through the “T” portion of the plate. These three screws tend to converge, so care must be taken when selecting screw length. A cortex screw is placed in the third tarsal bone in the load position and tightened. A cortex screw is placed into the proximal metatarsus in the load position and tightened. Locking screws are placed in the remaining plate holes. In the case of comminuted fracture of the small tarsal bones, the plate serves to buttress the joint and dynamic compression is not applied. In this case, cortex screws are used only to stabilize large fragments amenable to repair in lag fashion. Figure 6.4 Partial carpal arthrodesis. 4.5 mm narrow locking compression plates (LCPs) were applied dorsomedially and dorsolaterally. Axially unstable tarsal fracture and luxation of the proximal intertarsal joint are treated with a locking plate applied to the plantar lateral aspect of the tarsus. A 4.5 mm broad LCP is positioned to span the tarsus, extending from the calcaneus to the proximal third of the metatarsus, engaging the fourth metatarsal bone (Figure 6.5). Screw selection and placement varies and is predicated on the injuries being addressed. Horses treated with this method of fixation have returned to athletic capacity [7]. Figure 6.5 Distal tarsal arthrodesis. Locking head screws were placed through the horizontal portion of the locking T‐plate into the central tarsal bone. A cortex screw was placed in the third hole in the load position to compress the DITJ and TMTJ before inserting another locking screw in the third tarsal bone.
6
Principles of Locking Plate Applications in Large Animals
6.1 Principles
6.1.1 Comfort
6.1.2 Implant Geometry
6.2 Clinical Applications
6.2.1 Arthrodesis
6.2.1.1Proximal Interphalangeal Joint
6.2.1.2Metacarpo‐ / Metatarsophalangeal Joint
6.2.1.3Carpus
6.2.1.4Tarsus
6.2.1.5Cervical Vertebrae
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