The worldwide incidence of hip fractures is expected to increase to 2.6 million in 2025 and 4.5 million by 2050¹⁾; approximately 50% of cases will be femoral neck fractures (FNFs)^1,2). FNFs, their osteosynthesis, and postoperative rehabilitation have been associated with profound morbidity and socio-economic burden. In addition, osteosynthesis of the hip is frequently accompanied by development of significant fracture healing complications such as early implant failure, non-union, and avascular necrosis of the femoral head in up to 15% to 40% of cases³⁾. Fracture union after osteosynthesis of FNF is promoted by axial loading, which may lead to compression of the fracture ends, which could result in shortening of the femur neck. Even in the case of a united fracture, the functional outcome may be compromised due to shortening of the femoral neck or varus collapse resulting in limb length discrepancy, reduced offset, and limping^4,5). Thus, the implant of choice for fixation of an FNF can be challenging when making management decisions⁶⁾. Fixation of this type of fracture often involves the use of multiple cannulated cancellous screws (MCS) or a femoral neck system (FNS)^6-8). According to Stoffel et al.⁶⁾, FNS can provide increased rotational and angular stability and is twice as strong as MCS against shortening of the femoral neck and leg^6-8).

Discrepancies regarding the ideal surgical technique for management of an FNF provide the rationale for this trial, emphasizing the need to determine the performance of two different implants with the goal of reducing revision rates to a minimum. The purpose of this study was to determine whether there is a difference in femoral neck length in FNFs undergoing osteosynthesis with FNS or MCS at 1-year follow-up. In this study a comparison of femoral neck shortening after internal fixation of an FNF with FNS or MCS was performed.

1. Study Design

Ethical approval was obtained from the Institutional Ethical Committee (IEC) of All India Institute of Medical Sciences (AIIMS) (AIIMS/IEC/2021/3721) and Clinical Trials Registry, India (CTRI/2021/12/038871). The written informed consent was obtained from all patients. This was a prospective parallel interventional, single-center, CONSORT (Consolidated Standards of Reporting Trials)-compliant, single-blinded randomized controlled trial (Fig. 1).

Figure 1. CONSORT (Consolidated Standards of Reporting Trials) flow chart of the study. FNS: femoral neck system, MCS: multiple cannulated cancellous screws.

2. Eligibility Criteria for Participants

All patients between 18 years and 60 years of age undergoing osteosynthesis for management of subcapital or trans-cervical FNF presenting to the Orthopaedic Department were included. Patients presenting with basicervical fractures, intertrochanteric fractures, more than one fracture or any previous ipsilateral or opposite hip surgery or prior arthritis of the hip or inflammatory arthritis, a pathological fracture, or those who are smokers or tobacco chewers were excluded. Patients who refused to give consent were also excluded.

3. Settings for Data Collection

The study was conducted at a tertiary level trauma center in a University Teaching Hospital. Patients were enrolled consecutively between December 2021 and April 2022. All surgeries were performed by a single fellowship-trained surgeon (S.G.).

4. Interventions for Each Group

Once enrolled, patients were randomized and assigned to one of the two groups, based on the intervention used. The test group (FNS group) included patients undergoing fixation using the FNS (Synthes GmbH). Plates with a single screw hole were used for patients in the FNS group. The procedure was performed according to “Instructions for use” provided by the manufacturer⁶⁾.

The control group (MCS group) included patients undergoing fixation using Multiple Cannulated Cancellous partially threaded Screws (MCS) (Synthes GmbH). Three parallel MCS, arranged in an inverted triangle configuration were used.

5. Objective

The specific objective was to determine whether implant selection can influence neck shortening in patients with a FNF. We hypothesized that FNS would provide greater load to failure and less neck shortening than MCS.

6. Outcome

Baseline data including age, sex, body mass index, Charlson comorbidity index, side affected, fracture type as per Garden, and Pauwel’s classification was collected^9-12). Recorded details include time since injury, reduction quality, surgery duration, and follow-up. Standardized perioperative care was administered according to the institutional protocol. Garden’s alignment index was used for assessment of fracture reduction and graded as acceptable reduction (open or closed) or missed reduction¹³⁾.

The difference in femoral neck shortening between the FNS and MCS groups at the end of 1-year follow-up was measured to determine the primary outcome. The secondary outcome was to determine the correlation between neck shortening and patient reported outcome measures (PROMs). Femoral neck length was measured on a 15° internal rotation anteroposterior view of the pelvis on radiographs of both hips at 1-year follow-up. The difference was calculated from the normal opposite hip. A difference of 2 mm or more was considered significant. As reported by Nanty et al.¹⁴⁾ and Marmor et al.¹⁵⁾, assessment of radiographic parameters was performed by three independent observers six weeks apart who were blinded to the functional outcome (A.E., S.B., and S.G.)^14,15) (Fig. 2).

Figure 2. Pelvis radiograph showing parameters determined.

Settlement of disagreements was consensus-based. The quantitative parameters that were examined include femoral neck length, femoral neck shortening index, center trochanteric distance, neck shaft angle, lateral protrusion of screws in MCS, vertical offset, horizontal offset, and survivorship¹⁶⁾. Shortening and varus collapse was qualitatively stratified as severe (≥10 mm/10°) and not severe (<10 mm/10°). Scanning of radiographs was performed, and graphic software was used for measurement. The interclass correlation coefficient was calculated to determine inter-observer reliability¹⁷⁾.

Assessment of PROMs was performed using the modified Harris hip score (HHS) and the Patient-Reported Outcomes Measurement Information System (PROMIS) physical function (PF) and pain interference (PI) at three months and one year^18,19). HHS and PROMIS-PF were determined by an independent interviewer blinded to the radiographic outcome. Perioperative findings including duration of surgery from incision to skin closure, blood loss, number of image intensifier shoots, the length of hospital stay, and fracture union were also compared. Patients were asked to rate their functional satisfaction out of a score of 10 (0 to 4, not at all satisfied; 4 to 6, fairly satisfied; 6 to 8, moderately satisfied; and above 8, substantially satisfied). Fracture union was defined as clinically painless and radiographic continuity of trabeculae. Failure was defined as any loss of reduction or screw cut-out, cut through, implant back out or break between fragments, peri-implant fracture, loss of reduction, infection, or any change in implant position more than 2 mm from initial on subsequent radiographs or non-union.

7. Randomization

The Permuted-Block method was used for randomization. Block sizes ranging from 2 to 4 patients, each with an allocation ratio of 1:1, were made. A centralized 24-hour computerized randomization system, freely available in the public domain that allowed internet-based allocation, was used to ensure concealment. Randomization was performed once the patient was inside the operation theatre.

Single blinding was applied, although the surgeon was not blinded. Written informed consent was obtained from all patients.

8. Statistics

All information was collected in an electronic data capture system and checked for any illogical or mislaid data by an independent blinded reviewer. Logistic regression analysis was performed for comparison of the two groups. A P-value of <0.05 was considered statistically significant. Statistical analysis and minimal clinically important difference (MCID) was performed using statistical packages for R software version 4.3.1 (R Foundation for Statistical Computing).

Calculation of sample size was based on the primary outcome of femur neck shortening, as reported by Zlowodzki et al.⁴⁾. We determined that enrolment of 54 patients would give the trial 80% power to demonstrate a treatment effect at a two-sided alpha level of 5%, considering that the mean difference in the two groups is 20 points and the pooled standard deviation is 23 points. The target recruitment number was inflated to 60 to account for attrition (30 patients in each group).

From December 1, 2021, to April 30, 2022, out of 83 screened patients with a FNF, 60 patients were prospectively enrolled; 30 patients were randomly assigned to one of two groups—the FNS test group and the MCS control group. Twenty-three patients were not enrolled due to a deviation from the protocol involving ineligibility in inclusion criteria (n=19) and not following the informed consent process (n=4) (Fig. 1). Two groups were balanced with regard to baseline demographics. The mean±standard deviation age was 32.97±8.96 years in the FNS group and 32.70±7.72 in the MCS group (Fig. 1, Table 1).

Table 1 . Baseline Demographics of the Patients in Both the Groups.

Variable	FNS group (n=30)	MCS group (n=30)	P-value
Sex, female	18	16	0.74
Age (yr)	32.97±8.96	33.70±7.72	0.83
Charlson comorbidity index, median (Q1-Q3)	Q0: 26, Q1: 2, Q2: 1, Q3: 1	Q0: 28, Q1: 1, Q2: 1, Q3: 0	NS
Body mass index (kg/m2)	30.03±2.51	30.00±2.41	0.52
Pauwel’s classification (grading)	Type 2: 17, type 3: 13	Type 2: 18, type 3: 12	NS
Garden classification (type)	Type 2: 9, type 3: 13, type 4: 8	Type 2: 10, type 3: 14, type 4: 6	NS
Time from injury to surgery (hr)	29.47±15.22	32.87±16.08	0.43
Mechanism of injury	3 low energy; 27 high energy	3 low energy; 27 high energy	NS
Type of reduction	2 acceptable open; 1 missed; 27 acceptable closed	2 acceptable open; 28 acceptable closed	NS

Values are presented as number only, mean±standard deviation, or median only..

FNS: femoral neck system, MCS: multiple cannulated cancellous screws, NS: not significant..

1. Follow-up and Outcomes

Follow-up data for all outcomes were available through May 1, 2023. The median follow-up period was 13.6 months in the FNS group and 13.9 months in the MCS group (interquartile range, 13-15.4), and no patients were lost to follow-up for the primary outcome.

The primary outcome at the final follow-up, femoral neck shortening, was 3.77±1.87 mm in the FNS group, significantly lower when compared with the control group MCS, 6.53±1.59 mm. The interclass correlation coefficient for measurement of femur neck shortening was 0.80. In the FNS group, the mean abductor moment arm was reduced by 4 mm (range, 0-12 mm) compared to 7 mm (range, 0-16 mm) in the MCS group. The mean femur length was decreased by 5 mm (range, 0-15 mm) in the FNS group and 9 mm (range, 0-21 mm) in the MCS group. At the final follow-up, seven patients (23.3%) in the MCS group and three patients (10.0%) in the FNS group had severe shortening. Five patients (16.7%) in the MCS group and none in the FNS group had severe varus collapse. A moderate correlation of varus collapse with femur neck shortening was observed in the MCS group, while it was low in the FNS group (Pearson’s correlation coefficient 0.66; P<0.001) (Fig. 2, Table 2).

Table 2 . Postoperative Radiologic Outcome of Both the Groups Compared.

Variable	FNS group (n=30)	MCS group (n=30)	P-value
Neck length postoperative (mm)	44.63±2.90	42.03±2.65	0.035
Neck length normal (mm)	48.20±2.52	48.50±2.29	0.22
Neck length difference (mm)	3.77±1.87	6.53±1.59	0.031
Neck shortening index	0.078±0.039	0.134±0.031	0.045
Lateral implant protrusion (>5 mm)	0	8	0.029
CTD postoperative (mm)	62.15±2.81	59.25±4.24	0.023
CTD normal (mm)	67.14±4.34	68.74±3.15	0.34
CTD difference (mm)	5.15±3.32	8.21±4.33	0.041
NSA postoperative (°)	128.40±4.34	125.40±3.71	0.023
NSA normal (°)	131.60±2.72	130.70±4.25	0.25
NSA difference (°)	3.44±1.91	5.15±2.43	0.031
VO postoperative 1 year	49.42±3.61	46.12±2.14	0.039
VO normal	53.12±4.15	52.67±3.87	0.24
HO postoperative	44.14±3.18	43.31±4.72	0.34
HO normal	48.43±1.82	49.32±2.71	0.32
Neck shortening/varus (qualitative)	3 severe (10.0)	7 severe (23.3)	0.030

Values are presented as mean±standard deviation, number only, or number (%)..

FNS: femoral neck system, MCS: multiple cannulated cancellous screws, CTD: center trochanter distance, NSA: neck shaft angle, VO: verticle offset, HO: horizontal offset..

Lateral implant protrusion (>5 mm) was observed only in the MCS group (eight patients) (26.7%). The mean screw backout was 7 mm (range, 0-20 mm).

The secondary outcome, 3-month HHS, was higher in the FNS group, although no difference was observed at 1-year (93.66±5.52 in the FNS group vs. 91.81±3.38 in the MCS group). However, a downward trend of 1-year HHS with increasing neck shortening was observed in both groups. Although the 3-month PROMIS-PF was higher in the FNS group, no difference was observed at 1-year. No difference in the mean duration of hospital stay (3.32±2.8 days in the FNS group vs. 3.87±3.1 days in the MCS group) was observed (Fig. 3, Table 3).

Figure 3. Scatter plots for neck shortening and patient reported outcome measures (PROMs) in both groups. FNS: femoral neck system, MCS: multiple cannulated cancellous screws, HHS: Harris hip score.

Table 3 . Postoperative Outcome of Both the Groups Compared.

Variable	FNS group (n=30)	MCS group (n=30)	P-value
Duration of surgery, from incision to closing (min)	54.47±10.12	84.63±16.39	0.041
Blood loss (mL)	30.23±25.31	50.15±30.71	0.044
Effective radiation dose due to image intensifier shots (mSv)	0.89±0.77	1.17±0.54	0.032
HHS (3 months)	72.80±6.52	67.28±3.38	0.038
HHS (1 year)	93.66±5.52	91.81±3.38	0.56
Postoperative patient satisfaction (out of 10)	8.23±1.38	6.77±1.68	0.049
	2 Not at all satisfied 2 Fairly 9 Moderately 17 Substantially	5 Not at all satisfied 3 Fairly 9 Moderately 13 Substantially
Duration of hospital stay (day)	3.32±2.8	3.87±3.1	0.027

Values are presented as mean±standard deviation..

FNS: femoral neck system, MCS: multiple cannulated cancellous screws, Sv: millisievert, HHS: Harris hip score..

Bone union was observed in 28 patients (93.3%) in the FNS group and 27 patients (90.0%) in the MCS group. The mean union time was significantly shorter in the FNS group than in the MCS group (3.06±1.09 months vs. 3.89±1.28 months).

The intraoperative duration of surgery from incision to skin closure was significantly reduced in the FNS group by 17.7±7.1 minutes compared to the MCS group. Patients in the FNS group had less blood loss compared with those in the MCS group. The image intensifier shoots’ mean effective radiation dose was 1.17±0.54 mSv in the MCS group and 0.89±0.77 mSv in the FNS group.

2. Adverse Events

A total of seven adverse events (AE) were observed. In the MCS group, five patients had AE, one each peri-implant fracture, loss of reduction, implant failure, non-union, and infection. In the FNS group, two patients had non-union. Revision surgery was required for all patients. No significant difference in survivorship was observed in either group (Fig. 3, Table 4).

Table 4 . Postoperative Adverse Events of Both Groups Compared.

Variable	FNS group (n=30)	MCS group (n=30)	Type of fracture	Comminution	Missed reduction intraoperative
Peri-implant fracture	0	1	Garden 2 and Pauwel’s 2 type	No	No
Implant failure (breakage/bending)	0	1	Garden 4 and Pauwel’s 3 type	Postero-medial	No
Loss of reduction with implant in situ (postoperative)	0	1	Garden 4 and Pauwel’s 3 type	No	No
Non-union leading to surgical revision	2	1	Garden 3/4 and Pauwel’s 3 type in FNS Garden 2 and Pauwel’s 2 type in MCS	Postero-medial in one patient of FNS	Yes in one patient of FNS
Infection	0	1	Garden 2 and Pauwel’s 2 type	No	No
Readmission	2	5
Reoperation	2	5

FNS: femoral neck system, MCS: multiple cannulated cancellous screws..

The most significant finding of our study was that in young patients (<60 years) with FNF undergoing internal fixation using two different modalities, a significant difference in neck shortening but no difference in PROMs was observed between the FNS group and the MCS group at the end of the 1-year follow-up period. Despite recent publication of many studies and meta-analyses on the performance of FNS, no randomized controlled trials comparing the two procedures have been published.

The postoperative femur neck shortening index at 1-year postoperative differed significantly between the two groups, and use of FNS resulted in less neck shortening. This result might be explained by increased rotational and angular stability compared to MCS⁶⁾. Less shortening of the femur neck in FNS also makes it a length-stable implant; additional neck shortening could be prevented with use of an antirotation-screw locked into a bolt, making it a dynamic fixation model²⁰⁾. By contrast, MCS relies on friction between bone and the smooth portion of the cannulated screw interface, thus it cannot yield sufficient resistance to shortening²¹⁾. Despite this difference, even though the difference in early 3-month postoperative HHS was better in the FNS group, no statistically significant difference in the final HHS was observed between groups. This result contradicts the results of a Metanalysis reported by Patel et al.²²⁾, where significantly better functional outcomes were achieved with use of FNS. Better early 3-month postoperative HHS with FNS may be due to a shorter time to bone union and early weight bearing in patients with FNS. The scatter plot shows a negative correlation of femur neck length with PROMs which can be explained by findings from previously reported literature—i.e., a greater reduction in neck length is associated with poorer outcomes⁴⁾. Shortening the neck length can cause a limb length discrepancy, affecting the overall functional outcome.

Stoffel et al.²³⁾ reported that age rather than fracture pattern is predictive of the outcome of FNS. In contrast, our findings indicated that fracture pattern was more important than the patient’s age in young adults. In our study, the mean age was approximately 32.97±8.96 years in the FNS group compared to Stoffel et al.²³⁾, who reported a mean age of 65.8±18 years. In our study, a displaced vertical fracture with posteromedial comminution (Garden 4 and Pauwel’s 3 type) and intraoperative missing reduction in varus were reasons for reoperation in the FNS group²³⁾. Our study included more displaced vertical fractures (Garden 3/4 and Pauwel’s 3 type) (66%) than other studies (30.4%) because the patients included were young adults who presented with a high-velocity injury²³⁾.

Shorter surgery times and less blood loss was observed in the FNS group compared with the MCS group, in contrast to a retrospective study by Hu et al.²⁰⁾, which reported longer surgery time and more blood loss in the FNS group. The shorter duration of surgery from incision to closure reflects the ease of placing a single guide wire instead of three parallel wires in the MCS group.

The results of our study showed an AE rate of 6.67% at the end of a 1-year follow-up period, which showed good correlation with the predefined AE rate of 8.8% (4.5% to 15.2%) reported by Stoffel et al.⁶⁾, but considerably lower than that reported by Davidson et al.²⁴⁾ and Schuetze et al.²⁵⁾ (9.2% and 13.3%, respectively). A significant difference was observed in AE rates when compared with the MCS group (13.3%), which also showed good correlation with rates ranging from 9% to 30% reported in recent literature^16,26,27). AE rates in the lower confidence interval range were observed in both groups, as the drop-out rate from our study was minimal. The results of subgroup analysis of Pauwel’s type 3 fractures, which were 43.33% in the FNS group and 40% in the MCS group, showed that AE rates were 15.38% and 25%, respectively. This difference in outcome may be attributed to the biomechanical superiority of FNS⁶⁾.

Our study has many strengths and a few limitations. The strength of the current study is its prospective single-blinded randomized design and strict inclusion criteria. All patients were kept from follow-up. Functional assessment and implant hardware-related issues were considered. All procedures were performed by a single, fellowship-trained surgeon using the same surgical technique, which limits generalizability but confers internal study validity. Assessment of radiographic parameters was performed by independent observers blinded to the functional outcome to prevent measurement bias. As a high-quality trial, the results can be broadly applied to a larger patient population due to the small sample size, short follow-up period, and single-center study. A computed tomography scan was not performed at regular follow-up; thus, comparison of the bone union rates was difficult. All patients were younger than 60 years of age, thus the results do not apply to the elderly population. Another limitation is that the MCID, patient-acceptable symptom state (PASS), and Maximum Outcome Improvement Satisfaction Threshold score were not considered, because they have not yet been validated for the fracture neck of femur patients undergoing fixation. According to available literature, baseline MCID and PASS for 1 year-HHS (6.9; 84.8), PROMIS-PF (5.43; 47), and PROMIS-PI (–3.1; 53.7) were considered^28-30). No statistically significant differences in 1-year HHS, PROMIS-PF, and -PI were observed between the MCID and PASS in the two groups, whether or not the difference in symptom state is acceptable cannot be determined. There may have been “expertise bias” since the FNS was launched a few months earlier in the country, and there were only 47 cases (fewer than 50) at our institute prior to the start of the study. Our study provides long-term multi-centric follow-up for survival and maintenance of the results.

Significantly less shortening of the femur neck was observed in the FNS group compared with the MCS group. No statistically significant difference was observed at 1-year follow-up HHS in the FNS group compared to the MCS group. A downward trend of PROMs with increasing neck shortening was observed in both groups. Fracture patterns and intraoperative reduction were considered important in determining outcomes in young patients. The findings of this study suggest that FNS can be considered as a suitable alternative implant for internal fixation in young adults (<60 years) with trans-cervical and sub-capital FNFs.

No funding to declare.

No potential conflict of interest relevant to this article was reported.