For decades, open reduction and internal fixation performed using an extended extensile lateral approach has been the standard treatment for calcaneal fracture . This method achieves a certain degree of anatomic reduction, but the occurrence of serious complications has prompted the development of less invasive approaches . A minimally invasive sinus tarsi approach for anatomic reduction and stable fixation of complex calcaneal fractures has been described ; and in a study of 156 patients, percutaneous leverage, manual compression, and application of anatomic plates and compression bolts applied using a minimally invasive lateral approach were effective in the treatment of displaced intra-articular calcaneal fractures, with fewer soft tissue complications and good fracture reduction . However, the classic minimally invasive surgical method for calcaneal fractures—which involves fracture reduction, temporary fixation, and internal fixation with an implant—has certain disadvantages. Moreover, most surgical procedures require extensive intraoperative fluoroscopy, and the successful execution of each step depends on the surgeon’s experience, which can lead to suboptimal outcomes.
The approach used in minimally invasive surgery for calcaneus fracture is determined by the fracture pattern; thus, personalized surgical planning is critical . By CT imaging and use of a prototype produced rapidly by 3D printing, the surgeon can obtain detailed information on the fracture and plan a procedure that will yield satisfactory fixation . More importantly, the surgery can be simulated in vitro. Preoperative planning tends to be idealized and it is not possible to fully anticipate the surgical challenges in individual cases. Calcaneus fractures are complex , and there is no single treatment protocol that is suitable for all of the different types. The main point of the PSI was to guide the surgery according to the plan. We used a computer-generated model of the calcaneus in order to examine the features of the fracture  and simulate the reduction and fixation, then created a PSI to guide the actual surgery.
We set out to optimize the classic method of calcaneal fracture MIIF by making it more personalized and precise. To this end, we designed a surgical procedure based on a simulation and then performed PSI-assisted operation. The unique aspect of the present case is that the traditional calcaneal fracture MIIF procedure was modified so that it consisted of preoperative digital surgical simulation and preparation of the PSI, installation of Schanz pins (or K-wires) using the PSI, adjustment of the relationship between Schanz pins (or K-wires) and the PSI according to the surgical plan, and internal fixation with an implant using the PSI. Thus, the focus of the surgical procedure changed from internal conditions to external auxiliary tools, resulting in an operation that was better planned and more rapidly and accurately executed.
In this work we not only described a new type of PSI, but also demonstrated a novel method for internal fixation of calcaneal fractures. In other words, the whole surgery was PSI-assisted and the preoperative plan was executed step-by-step, which is fundamentally different from traditional surgery in which each step is improvised and is associated with a degree of uncertainty. In contrast, the procedure described in the present study is standardized and methodical and the whole operation can be improved or even modified, unlike the traditional PSI method in which only part of the procedure is optimized. Thus, our newly developed strategy can be used to accurately execute the preoperative plan. Indeed, the actual postoperative measurements of Böhler, Gissane, and calcaneus valgus angles and subtalar joint width (sustentaculum) did not deviate significantly from the preoperative plan, and the postoperative calcaneal volume overlap ratio with the preoperative design was 91.2%±2.3%.
The design of the guide plate and in particular, the correct location for installation of PSI part 2, is critical for successful fracture reduction surgery. CT scans not only reveal the bone but also the profile of soft tissues including skin. The internal profile of PSI part 2 was based on the skin profile of the operated area. However, 3 points are worth noting. Firstly, the internal profile of PSI part 2 was slightly larger than the skin profile of the surgical area, and we usually left a ~1-mm gap between them. Secondly, in order to minimize swelling in the surgical area and the effect on the skin profile, we improved PSI part 2 as follows: to accommodate any swelling, we left a larger gap in the area that did not correspond to the superficial bony marks at the closest and furthest ends of PSI part 2; and we designed PSI part 2 as 2 pieces of shell-like armor connected by a lock in a sliding slot at a limited distance that could accommodate soft tissue swelling. It is important to note that the distance of the slot prevented the 2 pieces of armor from shifting across the cross-section; in repeated computer simulations, this small movement did not affect the position of the Schanz pins (or K-wires). Thirdly, before the operation, we strictly advised the patient to adopt measures to minimize limb swelling such as raising the affected limb, using an ice compress, braking, and taking anti-inflammatory drugs.
The new operation method was simple and the surgery process was smooth, and a good postoperative effect was achieved. Moreover, the surgery time was just 28.16±10.70 min as compared to >60 min for classic calcaneal fracture MIIF and ORIF [17, 29]. The operation was guided at each step by the PSI, which facilitated the surgical procedure and eliminated the need for extensive intraoperative fluoroscopy, resulting in near-perfect fracture reduction and internal fixation.
Limitations of PSI-assisted MIIF of calcaneal fracture
The procedure described in this study has some limitations. Firstly, mastering new technology to design a reliable and effective PSI is a complex process that takes time and experience [30, 31]. In the first year of implementation, there were 3 cases (19 in total) in which the IFAU diverged from the preoperative plan. Specifically, the length of the cannulated screws required to fix the main body of the calcaneus axially during the operation deviated from the preoperative plan, which affected fixation of the fracture pieces or the functioning of the subtalar joint. In 2 cases, the screws were too long and penetrated the subtalar joint; and in another case, a screw that was too short resulted in fracture blocks that could not be firmly fixed. We have since performed more detailed tests and made improvements to the simulated preoperative plan; as a result, the situations just described never recurred. In our experience, the process of mastering this new technique takes about 1 year or 5 cases.
Secondly, the technique is not suitable for managing a fracture 72 h after injury. We used only Schanz pins (or K-wires) to reduce a fresh fracture; after 72 h has elapsed, closed reduction of an injury is not possible. Ideally, PSI-assisted surgery should be performed within 8 h after injury. Based on our experience, although PSI part 1 can facilitate the reduction of the fracture block within 72 h of injury, the optimal time to carry out the operation is within 8 h when there is minimal swelling of soft tissue, which is conducive to the installation of PSI part 2. However, it is difficult to carry out this surgical technique within 8 h if the surgeon is not sufficiently skilled in surgical design and 3D printing of the PSI takes a long time.
A third limitation of the present study is that this was a preliminary application of a method in a small sample of patients and short follow-up period. Therefore, prospective investigations with the large sample sizes and longer follow-up times are needed to evaluate the clinical applicability of PSI-assisted MIIF of calcaneal fractures.