**1. Introduction**

Arthrodesis has served as a standard treatment for painful arthritis of the ankle and the subtalar joints. The ankle joint is composed of the tibia and fibula and the subtalar joint includes the talus and calcaneus [1]. Fusion of the ankle, subtalar, and talonavicular joints together is known as pantalar arthrodesis and has been performed by orthopaedic surgeons since 1906 [2]. There also are non-surgical treatment methods such as non-steroidal anti-inflammatory medications and prefabricated or custom orthotics, but these methods are not effective in all patients.

Pantalar arthrodesis can be achieved with internal or external fixation. Internal fixation is preferred in most cases as it has many advantages over external fixations. The incidence of non-unions and mal-unions are reduced with internal fixation [3]. Implants that can be used for internal fixation are plates, screws, intramedullary nails, and wires. Locking compression plates (LCP) are advantageous when compared to others since they provide an angular-stable interface. LCPs are most commonly used for acute traumatic fractures, osteotomy fixation, non-union repair, and arthrodesis [4]. In the compression plating technique with conventional screws, primary stability is achieved by the compression between the plate and the bone. However, since there is no direct contact between the plate and the bone, the primary stability achieved by compression may be lost due to cyclic loading. The two major mechanisms for fixation failure in these constructs are axial tilting in the absence of the normal support given by bone due to fracture comminution and screw loosening in osteoporotic bone. In these cases, locking screws are preferred to conventional screw to limit these mechanisms of failure. Because locking screws lock into the locking plate, the amount of axial tilt and screw loosening is minimized. Proximal humerus internal locking system (PHILOS) (Figure 1) plating used in the pantalar joint utilizes the benefit that blood supply to the bone is preserved and soft-tissue injury minimized due to lack of compression. In addition, successful fixation result in anatomic reduction, stable fixation, preservation of blood supply, and early mobilization [5–9] which occur with the use of locking compression technology.

**Figure 1.** Submitted proximal humerus internal locking system (PHILOS) device for analysis; (**a**) plate through the thickness shows regions of interests marked by A and B and 1–3 investigated in this paper (**b**) the failed samples and (**c**) crack initiation and propagation of the plate.

A summary of device failures was compiled from literature [10–23] (see Appendix A). It identifies the devices, material of construction, failure modes and other fracture features. It is noteworthy that this may be the only case involving the failure analysis of PHILOS (Figure 1). A number of studies involving SS316L material revealed inclusion sites as crack origins [10,15–18], corrosion, wear and fatigue [10–13,15,16,18,19,21,23] with no visible mechanical failure ye<sup>t</sup> having failed clinically [12,14]. The majority of the devices were constructed with SS 316 and 316L; however, a few devices constructed with Ti-6Al-4V alloy [19–22] and pure titanium [23] were also found. From this summary it is prudent to investigate the chemical composition and inclusions that form pitting and corrosion fatigue related failures of the devices.

#### *1.1. Case Presentation (Clinical Summary)*

#### 1.1.1. Initial Presentation

A 68-year-old female was seen in the office for a right foot deformity. She experienced changes in ambulation over the past month with no history of trauma. Other than her progressive ankle deformity, she was relatively healthy.

On physical exam there was a fixed, non-reducible deformity of the right ankle. Palpable posterior tibial and dorsalis pedis pulses were present. Sensation over the dorsal and plantar foot was intact to light touch. Initial ankle X-rays were significant for a right ankle deformity consistent with chronic lateral subtalar dislocation with varus tibiotalar joint alignment (Appendix B, Figures A1 and A2). There was no evidence of bone loss or acute fracture. It was determined that the patient required a right ankle pantalar arthrodesis.
