Experimental and finite element method investigation of stapler and angle formations in joining (fusion) of the first metatarsophalangx joint

Hallux rigidus is a degenerative disease of the first metatarsophalanx (MTP) joint. It manifests itself in the form of pain and movement restrictions. It is the second most common pathological condition in the foot. İt is more common in women, it is seen in 2-2.5 % of people aged 50 and over. Although the exact cause of the disease is unknown, the majority of patients appear to have a positive family history. Hallux rigidus is divided into 4 stages. The symptoms are not excessive in the first stages, pain and limitation of movement increase in the final stages. There are several different treatment methods for hallux rigidus. The treatment method in patients admitted to the hospital is determined by the stage of the disease. On palpation in the diagnosis of the disease, osteophytes are frequently and there is tenderness in the dorsal joint. Determination is made by looking at the anteroposterior, oblique and lateral radiographs of the foot while the patient is in the grounded position. Flattening and enlargement of the metatarsal bone on oblique X-ray and a decrease in joint space can be seen as a result of flattening and enlargement of the metatarsal bone on anteroposterior and lateral X-ray. When viewed from the side, osteophytes can be observed at the head of the metatarsal bone and at the base of the proximal phalanx. There are two types of treatment methods for hallux rigidus deformation. In the initial stages, conservative treatment is recommended in order to stop the progression of the disease and reduce pain. In conservative treatment, orthoses, activity modifications, analgesics and corticosteroids can be preferred according to the pathological condition of the patient. Surgical treatment methods are recommended in cases where conservative treatment does not respond or in advanced stages of hallux rigidus. The primary aim of surgical treatment methods is to reduce pain and provide a comfortable walking for the patient. This option is not preferred for aesthetic purposes. In surgical treatment, there are many surgical treatment methods such as cheilectomy, resection arthroplasty, implant arthroplasty and arthrodesis. First MTP joint fusion is among the frequently used surgical methods. This method is generally preferred in advanced stages due to reasons such as loss of the joint function and nonunion. Kirschner wires (K-wires), stapler, plates and screws are preferred as implants in the first MTP joint fusion. Stapler have advantages such as being small in size, easy to apply compared to other implants, and being able to be used in osteoporotic bones. Stapler have become a frequently preferred method in first MTP joint fusion in recent years. In this study, the use of stapler in the fusion of the first metatarsal and proximal phalanx bones in hallux rigidus with the finite element method was investigated by experimental and numerical methods. For this purpose, the use of single stapler and two stapler together and the effects of the placement angles of the stapler were investigated. In the case of single stapler, the stapler were placed at an angle of 0° on the dorsal axis. In combinations where double stapler are used, the first stapler are placed at an angle of 0° on the dorsal axis, while the second stapler are placed in different combinations at an angle of 30°, 60° and 90° to the first staple. Stapler made of three different materials were tested in the experimental study. These are NiTi stapler made of super elastic and shape memory material, and TLX33 and TLX44 stapler made of stainless steel. In the experimental study, a force measuring device was designed to measure the force applied by the stapler into the circular canal to hold the bones together, and an experimental setup was designed to determine the force applied by the stapler to the bone in bone placement. The experimental setup, with one fixed jaw and movable jaw, was used to measure the force values formed in 3 different regions in the canal with the help of a force sensor by making the legs of the stapler open (parallel). Force measurements were carried out at room temperature (25°C) and body temperature (37°C) in order to determine the clamping force applied by the temperature-sensitive NiTi stapler made of shape memory material. When measuring at room temperature, the highest compression force was measured in TLX44 stapler, while the least compression force was measured in NiTi stapler. When the ambient temperature is brought to body temperature (37°C), the highest compression force was observed in NiTi stapler, while the lowest compression force was observed in TLX33 stapler. It has been observed that the force applied into the canal increases with the increase in temperature in NiTi stapler, which have super-elastic properties. There was no change in the temperature-dependent clamping force of TLX33 and TLX44 stapler. In the literature while designing the bone models, the first metatarsal bone and proximal phalanx bone lengths were determined as 60 mm and 35 mm, respectively, and their thickness was 10 mm. Staple placement distances are determined in single staple use and dual staple combinations, while stapler placed at 0° to the dorsal axis are designed to be 10 mm from the first metatarsal bone and proximal phalanx bone to the joint junction. When determining staple placement distances in double staple combinations, it is designed at 30°, 60° and 90° staple placement angles, 7 mm from the proximal phalanx bone to the joint junction and 13 mm from the metatarsal bone to the joint junction. In addition, while creating bone models, 3 different bone models were created as hallux angle (HA), 13° and dorsiflexion angle (DFA), 25°, 30° and 35° between metatarsal bone and proximal phalanx bone. These 3 different bone models are designed in 4 different ways, as 0° single staple combination on the dorsal axis and two staple placement combinations at 30°, 60° and 90° angles to the dorsal axis, according to the staple placement angle. A total of 12 different bone models were designed and the effects of staple placement combinations on mechanical strength were investigated. Bone models were produced in FDM type 3D printer using PLA material at 20% occupancy rate. Gyroid pattern was chosen as the filling pattern due to its similarity to the bone internal structure. Bone models produced by predicting different stapler placement angles were tested in the laboratory environment and the designed bone models were analyzed using the finite element method (in ANSYS Workbench software). The Young's Modulus (E) of the bones was defined as 20000 Mpa and the Poisson's ratio was 0.3. In reality, a variable force acts along the stapler feet. The greatest force acts on the tip of the feet of the stapler. In order to simulate this real situation in the analyzes, the force was defined to change linearly to the stapler feet. As a result of the tests and analyzes, the highest clamping force was measured in NiTi stapler. It has been observed that double staple combinations exert higher clamping force than single stapler. In double staple combinations, the highest clamping force was seen in the 0°-30° staple combination, while the lowest compression force was seen in the 0°-90° staple combination. However, more even distrubution seen 0-90 degrees staple combination. As a result, it was determined that the deformation increased when the stapler placement angles decreased. However, the lowest staple combination angles used in the study were 0°-30° and it was seen that this combination was not large enough to damage the bone structure. It was determined that the maximum stress and, accordingly, the compression force decreased as the stapler placement angles increased. It was concluded that in case of decrease in staple placement angles, the tension increases and accordingly the compression force in the joint area increases and provides more fixation.