**1. Introduction**

Numerous methods of fixation have been advocated for tibiotalocalcaneal (TTC) arthrodesis, including external fixation, screws [1], intramedullary nails [2], dynamic compression plate fixation [3], and the use of locking plates. Intramedullary nails have several advantages over other internal fixation devices, for instance, better bending and torsion strength [4,5]. However, this type of fixation may not be used with severe-deformity patients [6]. Plates are widely used in bone internal fixation. Different materials are used for medical implants, such as Ti-6Al-4V, 316l stainless steel, Co Cr. 316L stainless steel is used extensively in internal fixation devices due to its excellent properties, such as its competitive price point and compatibility with the human body. On the other hand, corrosion fatigue can drastically affect the lifetime of the internal fixation devices and lead to catastrophic failure.

As screw positioning is one of the main problems that leads to the failure of plates, locking compression plates (LCP) are preferred in arthrodesis. With this technique, the screw head threads are fixed to the plate with a specific angle that helps in reducing the risk of screw failure. Locking screws increase the stabilization of fractures, but they are more expensive than other types of screws. Hybrid fixation uses a combination of both locking and non-locking screws, which improve the stabilization economically. The locking and hybrid construct principles are outlined in Goswami et al. [7].

#### *1.1. Bone Screw Aspects of Design*

A screw comprises of a head, shaft, thread, and tip.


#### *1.2. Biomechanics of Bone Screws*

There are many differences among the cortical, cancellous, locking, and cannulated screws. The head of the locking screw has threads. Additionally, the size, core, and pitch of the threads change depending on the type of the screw. The cancellous screw has a larger pitch and a larger core diameter than the cortical screw. In addition, the tip of the screw is either rounded or fluted [10].

When using non-locking screws, a compressive force between the plate and the bone occurs with enough friction to provide the stability with a higher stress than the locking screws. On the other hand, the initial stress develops due to the tightening of the head inside the plate, followed by a compressive force that occurs on the bone around the locking screw. Additionally, the angle does not change between the locking screw and the plate that provides higher resistance against bending [11]. The cannulated screw has less strength than the non-cannulated screw as it has a larger core diameter [12]. The main mechanisms for screw failure are the excessive torque that leads to shear failure or the loading of the plate in a different direction than the screw, which leads to bending failure [9].

Studies showed that 42% of the torque that is applied to the screw is lost to overcoming the friction between the screw and the bone [9,13]. This is one of the main reasons that surgeons apply high force and torque, which leads to higher shear stress on the screw, where the shear stress is directly proportional to the torque and inversely proportional to the diameter. Shear failure force is proportional to the material ultimate shear stress and the thread shear area [14].

In addition, there are two main factors that affect the amount of torque required for the insertion of the screw, which are the tapping and lubrication. Perren [8] showed that 43% of torque loss happens due to the screw-plate friction, 42% of torque loss due to the screw threads-bone friction, and only 15% of torque transforms into an axial force. While, Hughes and Jordan [13] showed that 50% of torque occurred due to the screw-plate and screw-bone friction, 10% of torque loss is due to the screw threads-bone friction, 35% of torque loss is due to cutting threads, and only 5% of torque transforms

into an axial force. In contrast, the lubricated and tapped screw friction decreased by 50%, 0% of torque loss is due to the screw threads cutting, and 65% of torque transformed into axial force [9].

There are many designs of plates that can be used for tibiotalocalcaneal fixation or the fixation of a tibial bone fracture. Unfortunately, the lack of resources makes the best fixation option not always available. This issue has resulted in the use of humeral fixation plates for tibiotalocalcaneal arthrodesis. There are several studies that focused on the capability of using the humeral locking plate for tibiotalocalcaneal arthrodesis, as shown in Table 1, wherein 2007, a study on 18 patients showed that 94.4% of the patients had successful fusion after approximately 20 weeks when the proximal humeral locking plates were used for obtaining tibiotalocalcaneal arthrodesis [1]. Additionally, another study in 2011 reported successful results when the same plate was used for ankle arthrodesis [15]. In addition, in 2016, Shearman et al. showed that 81% of patients had satisfactory results when proximal locking plates were used for obtaining tibiotalocalcaneal arthrodesis [4].


**Table 1.** Studies focused on the capability of proximal humerus internal locking system (PHILOS) for tibiotalocalcaneal arthrodesis.

In the present study, a failed PHILOS construct was examined and a computational simulation performed to validate the regions of stress development causing the physical failure of the system.
