Patient-specific instrument-assisted minimally invasive internal fixation of calcaneal fracture for rapid and accurate execution of a preoperative plan: A retrospective study

Many doctors prefer to use minimally invasive surgery to treat freshly closed calcaneal fracture [1, 2]. In this procedure, the surgeon often performs closed reduction of the fracture or open reduction with a small incision, in conjunction with percutaneous fixation [3]. This has many advantages compared to open reduction internal fixation (ORIF) [4, 5] including fewer incision-related complications [6], shorter duration of limb swelling [7], shorter recovery time [8], and lower rate of fixation failure [9].

The classic minimally invasive surgical method for treating calcaneal fractures involves fracture reduction, temporary fixation, and internal fixation with an implant. However, this method has certain disadvantages. Firstly, the procedure requires extensive intraoperative fluoroscopy to observe the fracture and repeatedly examine internal fixation at each step of the operation [10]. Secondly, fracture recovery and internal fixation are frequently found to be less than ideal at follow-up, because the success of the procedure depends on the surgeon’s experience [11].

Digital surgical simulation can optimize the surgery process by increasing precision and personalization [12]. 3-Dimensional (3D) printing technology and patient-specific instruments (PSIs) have provided useful tools for orthopedic surgery [13]. Based on years of experience examining calcaneus biomechanics and using digital orthopedic technologies such as PSI-assisted surgery [14‐16], we developed an improved method of classical calcaneal fracture minimally invasive internal fixation (MIIF) by shifting the focus of surgery from internal conditions to external auxiliary tools. In order to achieve accurate anatomic reduction of calcaneal fractures in MIIF and prevent the occurrence of traumatic arthritis of the subtalar joint, we designed the new surgical procedure through digital surgical simulation and constructed a PSI for calcaneal fracture surgery. This allowed step-by-step execution of the preoperative plan, in contrast to traditional surgery methods. The new technique has been validated in numerous trials including a clinical study at our hospital, and has been approved by ethics committees. In the present retrospective study, we investigated whether PSI-assisted calcaneal fracture MIIF can rapidly and accurately execute the preoperative plan.

One patient underwent bilateral surgery. The mean intraoperative fluoroscopy time was 3.95 ± 1.78 h (range, 2–9 h). The condition of IFAU in 17 ft (16 patients) was the same as preoperative plan, but in 3 ft (3 patients) there was a discrepancy. The mean surgery time was 28.16 ± 10.70 min (range, 15–50 min). No patient developed complications (Table 2).

The Böhler angle in the preoperative plan was 31.5° ± 2.7° (range, 28°–35°), and the postoperative angle was 30.2° ± 5.9° (range, 20.6°–37.7°); the difference (5.7° ± 3.3°; range, 0.7°–12.2°) was not statistically significant (P = 0.407). However, the actual measured preoperative Böhler angle was 9.6° ± 6.5° (range, − 3.1°–18.4°), which differed significantly from the postoperative Böhler angle of 20.7° ± 7.0° (range, 9.3°–36.8°) (P = 0.000). The Gissane angle in the preoperative plan was 125.6° ± 4.0° (range, 118°–134°), and the postoperative angle was 123.6° ± 4.4° (range, 115.8°–128.7°); the difference between the 2 values (5.0° ± 4.3°; range, 1.0°–17.0°) was non-significant (P = 0.187). However, the actual measured preoperative Gissane angle was 151.3° ± 8.0° (range, 138.2°–161.5°), which was significantly different (P = 0.000) from the postoperative Gissane angle of 27.7° ± 7.7° (range, 14.0°–41.5°). The preoperative plan subtalar joint width (at the position of the sustentaculum) was 46.8 ± 1.3 mm (range, 44–48 mm), while the postoperative subtalar joint width was 46.4 ± 2.9 mm (range, 42.5–51.9 mm); the difference between the values (3.2 ± 2.0 mm; range, 0.1–7.9 mm) was nonsignificant (P = 0.718). However, the actual measured preoperative subtalar joint width was 55.4 ± 2.8 mm (range, 50.2–59.6 mm), which differed from the postoperative value of 9.0 ± 4.3 mm (range, 2.4–16.4 mm) (P = 0.000). The preoperative plan calcaneus valgus angle was 3.6° ± 0.8° (range, 3°–5°), and the postoperative angle was 3.3° ± 1.3° (range, 1.2°–5.6°); the difference of 1.4° ± 0.9° (range, 0.3°–3.6°) between the values was not statistically significant (P = 0.541). However, the actual measured preoperative calcaneus valgus angle was − 0.1° ± 5.0° (range, − 7.5°–10.9°), which differed (P = 0.010) from the postoperative angle of 4.6° ± 1.9° (range, − 5.3°–8.7°). The calcaneal volume overlap ratio in the preoperative design was 91.2 ± 2.3% (range, 87.0%–95.8%) (Table 3 and Fig. 7).

All patients had 3 postoperative follow-up examinations; the AOFAS score of all patients increased with time. The preoperative AOFAS score was 13.1 ± 4.7 (range, 5 to 18); and postoperative scores were as follows: 8 weeks, 62.5 ± 4.3 (range, 55–70); 6 months, 80.8 ± 7.6 (range, 71–92); and 1 year, 88.6 ± 5.5 (range, 81–97). The differences in AOFAS scores at each time point were statistically significant (P < 0.001) (Fig. 8).

For decades, open reduction and internal fixation performed using an extended extensile lateral approach has been the standard treatment for calcaneal fracture [21]. This method achieves a certain degree of anatomic reduction, but the occurrence of serious complications has prompted the development of less invasive approaches [22]. A minimally invasive sinus tarsi approach for anatomic reduction and stable fixation of complex calcaneal fractures has been described [23]; 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 [24]. 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 [25]. 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 [26]. 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 [27], 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 [28] 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.

We developed a surgical procedure for calcaneal fracture MIIF that is focused on external auxiliary tools rather than internal conditions as in the traditional method. Our results show that with PSI-assisted calcaneal fracture MIIF the preoperative plan can be accurately and rapidly executed, which can improve surgical outcomes.