Clinical outcomes of Ti-Ni shape-memory patella concentrator combined with cannulated compression screws in the treatment of C2 and C3 patella fracture: a retrospective study of 54 cases

The patella is the biggest sesamoid bone inside the human body, located between the quadriceps tendon and the patella tendon. As an important module in the knee extension system, the patella can function as a lever and increase the moment of quadriceps, enabling the extension of the knee and helping maintain the stability of knee joint [1]. The incidence of patella fracture in young people (18–49 years) is 0.01% [2]. What’s more, the incidence of patella fractures in adult including elder people is as high as 1.5%, accounting for 10% of all fractures [3]. Comminuted patella fractures (AO/OTA 34C2 and 34C3) are common challenges for orthopedic surgeons. Anatomical reduction and stable fixation are standard management to restore disrupted extension system and articular surface. Regardless of what kind of treatment was applied, there are still some patients who suffered poor outcomes from those comminuted patella fractures [4, 5].

While the operative techniques of patella fractures have undergone many changes over the past few decades, anterior tension band remains the most widely accepted method [6‐8]. It can transfer tension into pressure force to achieve dynamic compression between fragments, allowing early joint motion. However, the tension band still has many disadvantages [9, 10]. The traditional tension band is composed of smooth Kirschner wires (K-wire) and stainless wires. The smooth K-wires cannot provide direct fragmentary compression and the fracture may separate when the knee extends [11]. In addition, when the tension band is working, the stainless wires and K-wires may have stress break, wearing, loosening, migration and skin irritation. Some orthopedic surgeons have presented different modified tension band methods to avoid the shortcomings of traditional tension band, such as cannulated screws combined with stainless wires or cannulated screws combined with titanium cables. However, the early failures of tension band were still reported in the literatures with the rate of 12 to 30%, especially in comminuted fractures [12, 13].

Titanium-nickel (Ti-Ni) alloy is an one-way shape-memory alloy, which can be contoured in low temperature and reconverted to the previous shape in high temperature. Compared with traditional K-wires, the Ti-Ni shape-memory alloy has higher hardness, higher tenacity and better wearing resistance. What’s more, it has excellent corrosion resistance and desired histocompatibility [14, 15]. Based on these advantages, some Chinese factories have designed and manufactured patella concentrator out of Ti-Ni shape-memory alloy as an alternative approach for fixation of patella fractures. (Fig. 1).

During the past several years, we applied the special-designed Ti-Ni shape-memory patella concentrator (TNSMPC) combined with cannulated screws in the treatment of C2 and C3 patella fractures and conducted complete follow-up. In this paper, we presented our operation techniques. Additionally, we retrospectively collected and analyzed the existing clinical data of follow-up, evaluating clinical outcomes of this method.

Finally, 54 patients who accepted open reduction and internal fixation with TNSMPC combined with cannulated screws for comminuted patella fracture were evaluated in this study. All the cases were close fractures, without other concomitant injuries. Baseline demographic and clinical characteristics of the patients were summarized in Table 1. There were 31 males and 23 females. The average age of patients were 54.11 ± 11.24 years (range: 28–83 years). The average body mass index (BMI) was 23.02 ± 4.30 kg/m². Thirty-three fractures happened in the left side and 21 fractures happened in the right side. The injury mechanisms included traffic accidents (28 patients), sports (10 patients), works (9 patients) and activities in daily life (7 patients). According to the AO/OTA classification system, there were 23 C2 fractures and 31 C3 fractures. The average hospitalization was 14.67 ± 4.40 days. The average operation time was 77.5 ± 25.12 min and the average blood loss during the operation was 24.25 ± 4.70 ml (range: 20-40 ml).

All the patients got clinical healing 3 months after operation and bone healing 6 months after operation according to the results of radiographic and clinical evaluation. No broken, loosening, migration and skin irritation of the internal fixation were detected, which means excellent performance and histocompatibility of TNSMPC. The ROM for extension-flexion of the injuried knee improved significantly (P < 0.0001) from 108.91° ± 4.95° 3 months after operation to 124.28° ± 5.09° 6 months after operation and 136.11° ± 4.42° 12 months after operation, and closed to the uninjuried contralateral knee (146.42° ± 3.43°) (Fig. 3). The KOS-ADL was evaluated at least 6 months after the operation. The results showed only a few patients had symptoms severely influencing their daily activities (Table 2). Kneeling and squatting were limited for several patients and other functions were not difficult for patients (Table 3). SF-36 survey 1 years after the operation indicated the quality life of the patients were almost restored (Table 4). The results of role physical evaluation were relatively lower than other items, which indicated patients may need appropriate physical rehabilitation training respectively after the operation. According to the Bostman score, among all the 54 patients, 38 patients were excellent, 12 cases were good. The Fig. 4 showed a demo case.

The purpose of this retrospective case series was to evaluate the clinical outcomes of TNSMPC combined with cannulated compression screws in the treatment of comminuted patella fractures. The results revealed good clinical outcomes and few complications of this technique, which achieved high level of patient’s satisfaction. Due to the reliable fixation provided by the implants, patients were allowed to full weight bearing and rehabilitation after the surgery as soon as possible. All patients got bone healing 6 months after operation, with improved knee function, better life quality and few severe complications. In addition, the average operation time was 77.5 min and average blood loss was 25.5 ml, which means this operation method may not increase extra technique difficulties or tissue damages and it worth being promoted widely.

KOS-ADL and ROM are the most commonly used to evaluate the function of the knee after the surgery like arthroscopy, arthroplasty or fracture. The Bostman score reflects the patients’ subjective satisfaction to the treatments. Particularly, as a concise health questionnaire, SF-36 comprehensively summarizes the quality of life of the respondent from eight aspects: physiological function, physiological function, physical pain, general health, vitality, social function, role emotional and mental health. The vast majority of patients recovered significantly in terms of body function and quality of life after surgery in our study, while there are a small number of subjects who cannot kneel, squat or go down stairs easily after the surgery. We think it is a failure of patients to perform proper functional exercise after surgery and related to the fact that some patients themselves are too old and have poor mobility. Generally, the ROM of injured knee, function scores and life quality evaluation of patients indicated excellent recovery 1 years after surgery compared with the perioperative period.

Longitudinal K-wires combined with stainless wire in a figure-8 pattern was a classical fixation method for patella fractures and has been widely applied for decades. The cannulated compression screws can be applied to replace the K-wire without threads to provide direct fragmentary compression with the knee in extension [16]. However, the early failure of the internal fixation due to stress break, wearing, loosening, migration, electrolytic and skin irritation still exists, particularly for comminuted patellar fractures [12, 13]. To avoid intrinsic shortcomings of tension wiring, some new internal fixations have been designed and applied in clinical trials such as a fixed angle plate, a locking patella plate and TNSMPC in our study [17‐19]. The TNSMPC applied in our study was designed according to the anatomical characteristics of patella and was made of Ti-Ni alloy with high excellent corrosion resistance and desired histocompatibility. The TNSMPC can provide similar or even stronger dynamic compression between fragments like anterior tension band and can avoid the intrinsic defects of anterior tension band. The shape of the concentrator fits for the patella very well and the material is friendly to the soft tissue, reducing risk of skin problems. In addition, Ti-Ni alloy has excellent corrosion resistance and mechanical properties, which can provide stable fixation for comminuted fractures and avoid early stress break, wearing, loosening or migration. In terms of operation techniques, the install of the concentrator is much easier than that of stainless wire and the learning curve is short. After contoured in 0 °C sterilized normal saline and installed on the patella, concentrator was reconverted in 37 °C normal saline and clasped the patella like a talon strongly, without any extra damage of soft tissue. Other new fixations, such as fixed-angle plating and locking plate, need extra screws to fix the plate on the patella and have limitations in osteoporotic patients or severely comminuted fractures [20]. However, the TNSMPC can fix the fracture independently by its claws and causes no extra bone loss, which can achieve excellent outcomes in osteoporotic or comminuted cases. The study conducted by Yutong Zhang et al., described another similar patella concentrator and reported desired outcomes in pole fracture or comminuted patella fractures [21]. Fracture displacement due to the loosening of the claw is a potential complication of the operation while it did not happen in cases with comminuted fractures. Figure 5 shows the failure of the internal fixation in a case with a simple transverse fracture and the revision surgery was performed by removing the screws, reducing the fracture and fixing it with the claw and additional circle wire. Factors related to the complications are not so clear, which may include choosing inappropriate size of the claw, malposition of the implants, poor bone quality, knee flexion too early after the operation and so on.

Several limitations of our study exist and deserve mention. Actually, this is a retrospective case series without a control group and we didn’t compare the outcomes of our method with other osteosynthesis techniques directly, which affected the evidence level. We only chose comminuted fractures according to rigid standard in a single trauma center, so the number of cases are limited. Finally, the time of follow up was not so long. In the future, more cases from multiple trauma center and results of long-term follow up will be collected.

In summary, results of our study showed excellent outcomes of TNSMPC combined with cannulated compression screws in the treatment of C2 and C3 patella fracture. Without extra technique difficult or tissue damage, the TNSMPC and screws provide dynamic and static fragmentary compression respectively, ensuring stable fixation of comminuted patella fractures. Patients are allowed to early weight-bearing and rehabilitation without extra complications. As an alternative solution of traditional tension band, TNSMPC combined with cannulated compression screws is appropriate for comminuted patella fractures and worth spreading.