For decades, open reduction and internal fixation performed by an extended extensile lateral approach has been the standard treatment for calcaneal fracture . With this treatment, a certain degree of anatomical reduction is achieved, but the occurrence of serious complications has gained surgeons’ attention and resulted in the development of a minimally invasive approach . In recent years, with the efforts of scholars, minimally invasive surgery has become a more effectively surgical method solve calcaneal fractures. Nosewicz et al  reported sufficient exposure of complex calcaneal fractures by application of a minimally invasive sinus tarsi approach for anatomic reduction and stable fixation. Wang et al  applied a minimally invasive lateral approach in 156 patients and found that the percutaneous leverage, manual compression, and application of anatomic plates and compression bolts to be effective for displaced intra-articular calcaneal fractures, offering a combination of fewer soft tissue complications and good reduction.
But the classical minimally invasive surgical method for calcaneal fractures still has some disadvantages. The classical method includes fracture reduction, temporary fixation and internal fixation implantation. On the one hand, almost all of the above surgical procedures require a lot of intraoperative fluoroscopy. On the other hand, the surgeons determine the procedure at each step of the operation all depend on the surgeon's experiences and are often not optimal.
The minimally invasive surgery varies according to the fracture pattern. This makes personalized surgical planning important . With the help of CT imaging and a rapidly produced prototype, surgeons can obtain detailed information regarding the fracture and use software to plan a procedure that will result in suitable fixation . More importantly, the surgery can be simulated in vitro. Although preoperative planning is often near-perfect, it is not possible to fully anticipate the surgical challenges of individual cases. The calcaneus fracture type is complex , and there is no single treatment protocol that covers the various kinds of calcaneal fracture. The core point of the PSI was to guide the surgery as planned. we digitally reconstructed the calcaneus on the computer so that we could understand the characteristics of the fracture , second, simulating the reduction and fixation, then creating a PSI and using it to guide the actual surgery.
Our team intends to optimize the classical calcaneal fracture minimally invasive internal fixation into a more personalized and precision operation method. Therefore, we have creatively designed an optimal surgical procedure through the digital surgical simulation, and then we manage the operation through the assistance of PSI during the whole process. What is unique about this new case we reported is that the traditional calcaneal fracture minimally invasive internal fixation procedure was changed to: preoperative digital surgical simulation and preparation of PSI, install Schanz pins (or K-wires) by using PSI, adjustment of the relationship between Schanz pins (or K-wires) and PSI according to the surgical plan, internal fixation implantation by using PSI. In fact, we changed the operation procedure from traditional “focusing on internal conditions” to new “focusing on external auxiliary tools”, so as to optimize the surgery into a new surgical method that is more planned, faster and more accurate.
This paper not only shows a new PSI, but also shows an unprecedented new method for calcaneal fracture internal fixation. In other words, the whole process of the operation assisted by PSI, and performed the preoperative plan step-by-step, which is different from the traditional surgery method fundamentally. In fact, The steps of traditional method are improvisational, surgical procedures have some uncertainty. However, our new method of this study is standardized and relatively step-by-step. In addition, our new method is also very different from the traditional PSI method, which only optimizes a part of procedure, while our new method can improve the whole operation process, or even make transformation of the operation. According to our analysis, these characteristics should be the reason why the new technology can accurately execute the preoperative plan. It can be seen from Table 3 that the actual postoperative measurements of Böhler angle, Gissane angle, Subtalar joint width (sustentaculum) and Calcaneus valgus angle have no significant statistical difference from the preoperative plan, and the post-op calcaneal volume overlapping ratio with pre-op design was 91.2±2.3%.
The design of the guide plate is very important for the successful application of the technology. For PSI-part-2 in particular, whether it can be installed in the correct location is the key to accurate implementation of the technology. In fact, the CT data can see not only the bone, but also the profile of the soft tissue and, more specifically, the profile of the skin. The internal profile of the PSI-part-2 was based on the skin profile of the operating area. However, three points are worth emphasizing: First, the internal profile of PSI-part-2 is slightly looser than the skin profile of the surgical area, and we usually leave about 1mm gap between them. Second, in order to minimize swelling in the operation area increase or reduce the effect on the skin profile, we improved the PSI-part-2: On the one hand, to accommodate the swelling of the soft tissue, for the PSI-part-2, we leave a larger gap in the area that is not corresponding to the superficial bony marks or not at the nearest and farthest end of the PSI. On the other hand, we designed the PSI-part-2 as two pieces of shell-like armor; the two pieces of armor are connected by a lock in a sliding slot at a limited distance to allow for soft tissue swelling that can also hold the two pieces of armor together; it is important to note that this finite distance slot limits the two pieces of armor to be shifted across the cross-sections, and through repeated computer simulations, this limited movement does not affect the placement of Schanz pins (or k-wires). Third, before the operation, we strictly advised the patient to take measures such as raising the affected limb, ice compress, braking, taking anti-swelling drugs, etc., in order to minimize the swelling of the limb.
Based on the study, we found that the new operation method was very simple and the surgery process was smooth, and the postoperative effect was also very good; Moreover, it is worth noting that the surgery time using this new technique was 28.16±10.70 minutes in the study, while the surgery time of classic calcaneal fracture MIIF and ORIF are usually more than 60 minutes [17, 29]. With the assisted by PSI in whole process, the operation was guided at every step, surgical procedures became routine, intraoperative fluoroscopy was no longer required at every step, fracture reduction and internal fixation became relatively perfect. Therefore, it can be preliminarily believed that such a new type of technology will be expected to optimize the traditional surgical scheme greatly.
Limitations of the study
First, it takes time and experience to master this new technology because designing the PSI is a complex procedure [30, 31]. After all, surgeons and computer technicians need to accumulate experience to ensure the reliability and practicality of surgical procedures, but this takes time. In the first year of using the new technology, there were 3 cases (19 in total) occurred that the IFAU not same as the pre-op plan; The specific situation was that the length of cannulated screws actually used to fix the main body of the calcaneus axially during the operation was different from the pre-op plan, and this difference affected fixation of the fracture pieces or function of the subtalar joint. More specifically: In two cases, the pre-op design length of each cannulated screws was too long and entered the subtalar joint; In another case, the pre-op design length of a cannulated screw was too short, resulting in fracture blocks cannot be firmly fixed. Since then, we have done more and more detailed tests and made improvements in the pre-op simulated operation plan, and the situation has never happened again. As a result, the learning process of master this new technique takes about one year or five cases in our experience.
Second, the technique is not suitable for manage a fracture 72 hours after injury. We just use the Schanz pins (or K-wires) to reduce fracture; fresh fracture is easy to reduce while a fracture 72 hours after injury can’t be reduced with closed reduction. If there are conditions, we think it is best to conduct the PSI assisted surgery within 8 hours after injured. In particular, it is important to note that, according to our experience, although the reduction of the fracture block within 72 hours after the injury is smooth with the help of PSI-part 1, but the best time to carry out the operation is within 8 hours of the injury. The reason for this is that the change of soft tissue swelling is minimal within 8 hours of the injury, which is most conducive to the installation of PSI-part 2. However, it should be pointed out that if the surgeon is not skilled in surgical design and the speed of 3D printing PSI is slow, it is difficult to carry out this surgical technique within 8 hours of injury. Therefore, this disadvantage is not friendly to doctors who are just beginning to develop the technique.
Third, this is preliminary application with the small number of sample sizes and short period of follow-up. Therefore, further prospective investigations with the large numbers of sample sizes and longer follow-up duration will be necessary to investigate appropriate values for clinical application.