Patients who underwent neural exploration of injured CPN from January 2009 to January 2019 were included in this retrospective cohort study which was approved by the Ethics Committee of our institution. All the subjects signed the informed consent form. Inclusion criteria consisted of (1) patients with open CPN injuries, (2) patients with closed fracture who had clinical symptoms of CPN injury and underwent surgical exploration during the treatment of fracture, (3) patients with definitive CPN injuries without improvement after 3 months of conservative treatment, and (4) patients with follow-up period more than 12 months after surgery. Exclusion criteria were constitutive of (1) patients with incomplete medical record, (2) patients with follow-up period less than 12 months or loss of follow up after treatment, (3) patients with diabetic peripheral neuropathy, (4) patients presented with foot drop caused by central nerve disease, (5) patients with definitive CPN injuries without motor dysfunction, and (6) patients underwent ankle arthrodesis or amputation due to severe trauma.
A total of 568 patients with injured CPN were admitted in our hospital from January 2009 to January 2019, of which 181 cases were excluded from the study based on our exclusion criteria. The remaining 387 patients were divided into control group with good result (n = 320) and case group with poor result (n = 67) according to the BMRC grading system according to the last follow-up evaluation[18, 19] (Fig. 1).
The basic information of patients was collected which included age, sex, living area (village and city), occupation (manual and mental work), educational background (low and high level), medical history (diabetes, hypertension and cardiovascular disease), drinking history, smoking history, weight and height. Patients with college or higher education background were defined as high education level, while the others were defined as low education level. The formula (BMI = weight (kg) /height (m2)) was used to calculate the body mass index (BMI). Patients with BMI lower than 18.5 were defined as thin, between 18.5 < BMI < 24 kg/m2 as normal, BMI between 24 and 28 kg/m2 as overweight, and BMI ≥ 28 kg/m2 as obesity.
Factors related to the CPN injuries were also collected which include the injured side, etiology of injuries, duration of symptoms and innervated muscle strength. Etiology of injury[5, 21, 22] consisted of (1)scar formation, including skin scar and posttraumatic scar formed by connective tissue, (2) knee injuries, including direct or indirect trauma, open injuries, knee dislocation and fracture of the fibular head and tibial plateau, (3) anatomic factors, which caused secondary entrapment due to a fibrous band at the origin of the peroneus longus, (4) external compression sources, for example, tight splint/cast and compression wrapping/bandage, (5) iatrogenic injury from hip arthroplasty injury or knee arthroplasty injury, (6) hip fracture, including acetabular fracture, femoral neck fractures and intertrochanteric fracture, and (7) vascular injury, caused by femoral artery embolization or popliteal embolization.
Laboratory results on the day of admission were collected and analyzed. White blood cells (WBC), red blood cells (RBC), hemoglobin (HGB), and platelets (PLT) were performed by an automated hematology analyzer (SYSMEX 2000; Sysmex Corp., Kobe, Japan). Total protein (TP) and albumin (ALB), triglyceride (TG), total cholesterol (TC), high-density lipoprotein cholesterol (HDL-c), low-density lipoprotein cholesterol (LDL-c), apolipoproteins A1(Apoa1), apolipoproteins B (Apob), blood glucose (Glu), serum potassium ion(K), serum sodium(Na), serum calcium(Ca), serum phosphorus(P) and serum magnesium(Mg) were measured by the Dimension AR/AVL Clinical Chemistry System (Newark, NJ, USA) and its supporting reagents. All of tests were operated in the clinical laboratory of our institution.
The CPN was explored under the operating microscope. According to the intraoperative findings, the external compression factors causing the nerve entrapment were released completely. Patients with severe adhesion among nerve bundles caused by scar or hematoma were treated with endoneurolysis. Patients with obvious edematous nerve, blurred or disappeared neurovascular network were treated with epineural neurolysis. The released peroneal nerve was placed in a soft tissue bed with enough blood supply. Neuroma was resected and end-to-end neural anastomosis was performed. Soleus muscle branch of the tibial nerve was transfer to the distal CPN when nerve defect could not be directly sutured even under joint flexion position. Patients with atrophy of anterior tibial muscle or extensor digitorum longus muscle were treated with posterior tibial tendon transfer at the same time of neurolysis. Patients with shortened Achilles tendon with malformation, were treated with Achilles tendon lengthening at the same time of neurolysis. According to Classification of Nerve Injuries described by Sunderland[18, 19], the injured nerves was divided into five types in line with the intraoperative observation.
Plaster immobilization of the knee joint was performed in patients with unstable fracture, neurorrhaphy or soleus muscle branch transfer for 4 weeks after operation. And, plaster fixation in dorsal extension of ankle was performed in patients with Achilles tendon lengthening or posterior tibial tendon transfer. All the patients were treated with oral Vitamin B for 3 months and underwent physiotherapy.
Muscular strength of ankle dorsiflexion and toe dorsal extension was assessed according to the British Medical Research Council (BMRC) scoring system at patient’s last visit. The clinical outcomes were categorized as poor (case group) if the muscular strength of ankle dorsiflexion or toe extension ≤ M2, and as good (control group) if the muscular strength ankle dorsiflexion and toe extension ≥ M3 .
The collected data were independently recorded, verified and corrected by two staff members using EpiData 3.1 software (EpiData Association, Odense, Denmark). Statistical analyses were implemented by R Studio (Version 1.2.5001) with rms, ROCR, gplots and forestplot packages. Variables included were tested for normality, and the skewed distribution variables were transformed by natural logarithm. The continuous variables, analyzed using the Student t test, were expressed as mean ± standard deviation (SD), and the count variables, detected using the Chi-square or Fisher’s test, were expressed as number (%). Two-tailed analysis with p-value < 0.05 was considered as statistically significant level.
Taking outcomes as bivariate dependent variables and the other factors as independent variables, we used univariate logistics regression analyses to assess which explanatory variables are associated with recovery. Variables with statistical significance were fitted to regression model 1. We incorporated the variables of p-value (< 0.1) in univariate analysis into multivariate logistic regression analysis by stepwise method. The variables with low contribution (p-value < 0.05) to the model were eliminated by stepwise selection method. Reserved variables were fitted to regression model 2. Better model selected by sensitivity analysis was used to develop the nomogram. The neural function recovery discrimination was assessed by the area under the curve (AUC) of the receiver operating characteristic (ROC) curve.