HIGHLIGHTS • HO in the knee joint after reamed IMN is not uncommon and was observed in 15% of the cases. • No differences were noted in HO formation between suprapatellar and infrapatellar nailing of tibial fractures. • HO in the knee after IMN has a limited clinical effect on patients during short to mid-term follow-up. • Thorough joint irrigation after IMN is encouraged because postoperative reaming debris is markedly associated HO formation in both the infrapatellar and suprapatellar approaches.

Tibial fractures represent the most common long bone fractures with an estimated incidence of 500,000 per year,1 and tibial shaft fractures occur in approximately 20 patients per 100,000.2 Intramedullary nailing (IMN) has become the mainstay of treatment of these injuries, and the practice of reamed nailing has demonstrated favorable results, more prominently in closed tibial shaft fractures.3–6 While historically, research has mainly scrutinized the union rate and time to healing among different approaches to tibial nailing, little attention has been paid to knee HO, a potential cause of postoperative knee pain and functional loss of motion.

Heterotopic ossification (HO) is defined as the formation of extraskeletal bone in soft tissues and muscles. The diverse pathologic process can be theorized as aberrant tissue repair, leading to ectopic bone formation through stages of dysmorphic calcification before eventual ossification occurs.7,8 Dependent on the temporal stage of HO development, the clinical presentation can range from asymptomatic to immobilizing ankylosis across joints, causing notable morbidity.7 HO is a commonly recognized complication in the aftermath of traumatic injury and treatment thereof, charting peak incidences of up to 100% after surgical treatment of acetabular fractures, hip arthroplasty, and antegrade femoral nailing.9–11 However, it is discussed at a lesser frequency around or inside the knee joint, although most recent reports mention a frequency of 34% to 37% in patients with a knee dislocation12,13 and 14% to 39% in patients treated with total knee arthroplasty.14–16 While intra-articular and periarticular HO in the knee has been reported after retrograde IMN of femoral shaft fractures,17,18 antegrade tibial nailing introduces a similar surgical insult to the knee joint, which can be reflected in overall decreased functional outcome and knee pain after surgery.19 However, the possible role of HO in post-treatment outcome has not been discussed thus far, and to date, no estimated frequency or associated factors have been reported for the occurrence of HO after reamed tibial nailing. Moreover, the role of retained bony reaming debris and its potentially permissive effect on HO development has not been studied. Therefore, the aim of this investigation was to conceptualize the role of periarticular knee HO in tibial IMN by (1) assessing the frequency and (2) evaluating for potential associations with HO development after reamed tibial nailing, including its association with knee pain and range-of-motion (ROM) deficits postoperatively. We hypothesized that HO would be more frequent in the suprapatellar approach because this approach results in direct violation of the quadriceps tendon and knee joint itself. Furthermore, the sequential passage of reamers across from the entirety of the joint would allow more opportunity for seeding of reamed, ectopic bone. We theorized that HO development would be associated with risk factors similar to those reported in hip HO.

Methods Study Design

In this institutional review board–approved study, a retrospective analysis of all adult patients who underwent tibial IMN for a tibial shaft fracture (OTA/AO classification: 42A/B/C20) at an urban level 1 trauma center over the course of 4 years (January 1, 2015, to December 31, 2018) was conducted. Preoperative and postoperative radiographs of the tibia and knee were evaluated for development of HO in the knee joint. Choice in nailing technique for tibial IMN (suprapatellar and infrapatellar) and those with a minimum radiographic follow-up of 6 weeks were included.

Chart Review and Patient Demographics

A detailed chart review was performed to obtain demographic data (age, sex, and race) as well as injury characteristics and hospital utilization, including Injury Severity Score (ISS); head, chest, and spinal injury; associated burns; intensive care unit (ICU) admission; ICU length of stay; and requirement and length of mechanical ventilation. Injuries to the head, chest, and spine were classified as serious or severe in accordance with the Abbreviated Injury Score.21 Charts were also reviewed for postoperative ketorolac use to control for the potential moderating effect of nonsteroidal anti-inflammatory drugs on the development of HO.7 Clinical data on the last follow-up were evaluated for knee pain using the visual analog scale and ROM as dictated by attending staff physicians. Surgical reports were reviewed to extract the surgical approach (suprapatellar versus infrapatellar approach) and concomitant ipsilateral knee injuries requiring surgery (ipsilateral tibial plateau injury, ipsilateral distal femur injury with intra-articular extension, and femur fractures treated with retrograde IMN).

Radiographic Analysis

At our institution, all patients undergoing surgical fixation undergo immediate postoperative imaging with flat plate radiographs in the postoperative care unit. These immediate postoperative radiographs were scrutinized for the presence of postsurgical retroinfrapatellar reaming debris, adjacent to the cortical nail insertion point. This was categorized as a binary variable (yes/no) under direct comparison with preoperative radiographs. Cases were categorized as “yes” if there was evidence of retroinfrapatellar reaming debris (Figure 1). Second, as the primary outcome, images obtained on last follow-up were reviewed for heterotopic bone formation within the anterior knee compartment adjacent to the cortical IMN entry site. By precedence of previous reports describing HO in and around the knee joint,13,22–24 as well as most frequently encountered examples during a local pilot sample at the local institution, HO was categorized as (A) retropatellar fat-pad calcification, (B) ossified islands, and (C) tibial bone spurs (Figure 2, A–C). Radiographic evaluation of both time points (postoperative versus last follow-up) was conducted by two independent observers (J.S., H.M.P.) for interobserver reliability. The intraclass correlation coefficient was used for interobserver reliability.

Figure 1:

Postoperative plain radiograph of the knee (lateral view) demonstrating retained retro/infrapatellar reaming debris (green arrow).

Figure 2:

Anterior-posterior and lateral plain radiographs of (A) retro/infrapatellar fat-pad calcification, (B) ossified islands, and (C) tibial bone spurs (green arrows).

Statistical Analysis

Descriptive statistics are displayed as mean (range) for continuous variables and frequency (percentage) for categorical variables, which were examined in descriptive histograms and box plots. The assessment of normality in the data set was performed using the Kolmogorov-Smirnov test. For not normally distributed variables, nonparametric testing was performed. Pearson chi-square test was conducted to evaluate frequency of distributions of HO regarding surgical approach. Multivariable logistic regression analysis was conducted to analyze the correlation between HO occurrences and collected variables. Odds ratios (ORs) were set at 95% confidence intervals (CIs). Statistical significance was determined as a P-value of less than 0.05 and power of 0.8, and all statistical analyses were performed in SPSS version 28 (IBM).

Results Patient Characteristics

Two hundred thirteen patients (19.7% female, 80.3% male), 39.25 ± 15.83 years, met inclusion criteria. Seventy percent of patients were Black or African American, 22% White, 5% Hispanic, 1% Asian, and 2% were of a race not further specified. Demographic characteristics are listed in Table 1.

Table 1 - Patient Demographics Variable %, unless otherwise noted Total no. of patients (n) 213 Sex, %  Male 80.3  Female 19.7 Age in yr, mean (SD) 39.25 (15.83) Race, %  Black or African American 70  White 22  Hispanic 5  Asian 1  Other 2 Body mass index in kg/m2, mean (SD) 27.20 (6.71) Length of surgery in min (mean, SD) 71.82 (44.72)  Prolonged procedure duration (≥2 hr) 6.9

One hundred forty-eight patients (69.5%) underwent reamed suprapatellar nailing, and 65 patients (30.5%) underwent reamed infrapatellar nailing. The decision on surgical approach was at the discretion of nine attending orthopaedic trauma surgeons included in this analysis. The average ISS was 12.18 ± 9.67. Thirty-seven patients (17.4%) were admitted to the ICU for an average of 2.53 ± 11.31 days, and 15 patients (7%) required mechanical ventilation (0.9 ± 4.5 days). Concomitant injuries included burns (n = 7) and serious head (n = 9), thorax (n = 21), and spine (n = 1) injuries. Eight patients also had ipsilateral tibial plateau fractures, 11 had supracondylar distal femur fractures, and eight were treated with an ipsilateral retrograde femoral nail (Table 2).

Table 2 - Univariate Analysis of Injury Characteristics and Potential Risk Factors Per Surgical Approach Variable Overall Suprapatellar approach Infrapatellar approach P n = 213 n = 148 n = 65 Injury Severity Score, mean (SD) 12.18 (9.67) 13.09 (10.33) 10.20 (7.72) 0.076 Concomitant injury, n (%)  Head injurya 9 (4.2) 9 (6.1) 0 0.06  Chest injurya 21 (9.9) 16 (10.8) 5 (7.7) 0.482  Spinal injurya 2 (0.9) 1 (0.7) 1 (1.5) 0.518  Associated burns 7 (3.3) 5 (3.4) 2 (3.1) 0.92 Ipsilateral knee injury, n (%)  Tibial plateau fracture 8 (3.8) 5 (3.4) 3 (4.6) 0.702  Distal femur fracture (intraarticular) 11 (5.2) 6 (4.1) 5 (7.7) 0.441  Femur fracture with retrograde IMN 8 (3.8) 4 (2.7) 4 (6.2) 0.251 Hospital utilization, n (%)  ICU admission 37 (17.4) 28 (18.9) 9 (13.8) 0.368  Mechanical ventilation 15 (7) 11 (7.4) 4 (6.2) 0.737 Postoperative ketorolac use 6 (2.8) 4 (2.7) 2 (3.1) 0.205 Retro/infrapatellar reaming debris, n (%) 52 (24.4) 39 (26.4) 13 (20) 0.32

ICU = intensive care unit, IMN = intramedullary nail

a“Serious” or “severe” according to Abbreviated Injury Score (AIS).

Heterotopic Ossification

On final follow-up (average: 41.43 weeks, range: 6 to 235 weeks), HO was recorded in 32 cases (15%). Of those, 12 (5.6%) were classified as retropatellar fat-pad calcification, 11 (5.2%) were ossified islands, and 8 (3.8%) were tibial bone spurs. Postsurgical retroinfrapatellar reaming debris was seen in 52 cases (24.4%) on immediate postoperative imaging (Table 2) without significant differences between surgical approaches (suprapatellar: 26.4%, infrapatellar: 20%, P = 0.32). Of those 52 cases, 17 (32.7%) developed evidence of HO on final follow-up. Furthermore, logistic regression analysis demonstrated that patients with retroinfrapatellar reaming debris had approximately 5× higher odds of developing HO on final radiograph (OR, 4.73; CI, 2.15 to 10.34) (Table 3). No statistical difference was observed between approach and retroinfrapatellar reaming debris (P > 0.05) (Table 2). In addition, ISS (OR, 1.05; CI, 1.01 to 1.09), ICU admission (OR, 2.89; CI, 1.20 to 6.82), chest injury (OR, 3.4; CI, 1.31 to 9.62), and ipsilateral retrograde femoral IMN (OR, 5.08; CI, 1.11 to 22.01) showed significant association with HO on last follow-up (Table 3). No association was observed between HO and chosen nailing technique (P > 0.05). Average knee pain (1.37 ± 2.73) and decreased knee ROM, defined as less than 120° arc of motion (n = 45, 21.1%), on final follow-up were also not significantly associated with HO development. Postoperative ketorolac use showed no significant correlation with HO (P = 0.598). The intraclass correlation coefficient for radiographic evaluation was 0.924, indicating excellent inter-rater reliability.25

Table 3 - Regression Analysis of Potential Risk Factors for Heterotopic Ossification Variable Odds ratio (95% confidence interval) P Suprapatellar approacha 1.04 (0.46-2.35) 0.922 Injury Severity Score 1.05 (1.01-1.9) 0.017 Head injury 1.066 (0.33-8.36) 0.54 Chest injury 3.40 (1.31-9.62) 0.037 Spinal injury 2.30 (0.14-37.30) 0.559 Associated burns 0.91 (4.81-0.72) 0.91 Ketorolac use 4.57 (0.41-53.34) 0.468 Tibial plateau fracture 0.8 (0.1-6.75) 0.839 Distal femur fracture (intraarticular) 3.53 (0.97-12.85) 0.056 Femur fracture with retrograde IMN 5.08 (1.11-22.01) 0.018 ICU admission 2.89 (1.20-6.82) 0.042 Mechanical ventilation 1.46 (0.39-5.48) 0.578 Retro/infrapatellar reaming debris 4.73 (2.15-10.34) <0.001

ICU = intensive care unit, IMN = intramedullary nail

aWith reference to infrapatellar approach.

Subgroup Analysis

To minimize potential confounding through retained reaming debris, cases without evidence of retained retroinfrapatellar reaming debris were analyzed separately and demonstrated no notable change in correlations. Of 161 patients without retained postoperative reaming debris, 15 (9.3%) developed HO. ISS (OR, 1.06; CI, 1.01 to 1.12), ICU admission (OR, 4.45; CI, 1.47 to 13.43), chest injury (OR, 4.06; CI, 1.12 to 14.72), distal femur fracture with intra-articular extension (OR, 7.05; CI, 1.5 to 33.15), and ipsilateral retrograde femoral IMN (OR, 2.54; CI, 1.27 to 24.28) showed significant associations with HO on last follow-up (Table 4). Furthermore, there was no significant difference in the distribution of HO between suprapatellar (10.1%) and infrapatellar (7.7%) approaches (P = 0.776).

Table 4 - Regression Analysis of Potential Risk Factors of Heterotopic Ossification in Patients Without Retained Reaming Debris Variable Odds ratio (95% confidence interval) P Suprapatellar approacha 0.74 (0.23-2.45) 0.625 Injury Severity Score 1.06 (1.01-1.12) 0.021 Head injury 1.42 (0.16-12.38) 0.752 Chest injury 4.06 (1.12-14.72) 0.033 Spinal injury — — Associated burns 1.05 (0.93-11.84) 0.969 Ketorolac use 1.38 (0.49-3.89) 0.545 Tibial plateau fracture 3.41 (0.33-34.95) 0.302 Distal femur fracture (intraarticular) 7.05 (1.5-33.15) 0.013 Femur fracture with retrograde IMN 2.54 (1.27-24.28) 0.016 ICU admission 4.45 (1.47-13.43) 0.008 Mechanical ventilation 2.09 (0.41-10.59) 0.372

ICU = intensive care unit, IMN = intramedullary nail

aWith reference to infrapatellar approach.

Discussion

This study aimed to identify the incidence and potential risk factors associated with HO in the knee after antegrade reamed IMN of tibial shaft fractures. Our results demonstrate a radiographic incidence of HO in 15% of patients undergoing tibial nailing at an average follow-up time of 42 weeks postoperatively. HO development was associated with retained postoperative reaming debris, increased ISS, ICU admission, chest injury, and ipsilateral retrograde femoral nailing at the time of index surgery. The study did not find an association between radiographic HO formation with subjective knee pain or notable ROM deficits.

The suprapatellar nailing technique has gained widespread popularity, in part, because of relative technical ease of imaging and improved functional alignment, when compared with the infrapatellar approach.26 In this study, the suprapatellar technique was predominately used. Opponents of the suprapatellar approach may argue that this approach violates the knee joint and can cause more damage to intra-articular structures than infrapatellar approaches (ie, retropatellar fat pad) because of its path of instrumentation. Furthermore, this intra-articular path of instrumentation should theoretically allow more opportunity for aberrant deposits of reamed bone within the intra-articular space and/or surrounding tissue and, therefore, could be associated with a higher incidence of HO. However, our results demonstrated no correlation with HO development between approaches. Similarly, the distribution of retained retroinfrapatellar reaming debris was not markedly different between the two studied surgery techniques. These findings add to the accumulating literature of the comparable risk profiles associated with both the infrapatellar and suprapatellar approaches for tibial nailing.

Knee pain on last follow-up was recorded at a frequency of 25% after suprapatellar nailing and 25.8% after infrapatellar nailing, which was similar to that reported in the previous literature.19 In this study, patients who were pain-free at their last follow-up demonstrated evidence of HO less frequently (13%) than those who solicited knee pain on their last follow-up appointment (17%); however, this did not reach statistical significance (P = 0.273), nor were there any cases of symptomatic HO requiring surgery. However, readers are encouraged to interpret this finding with caution because this is a descriptive study that cannot provide statistically valid information for clinical outcomes associated with radiographic findings. Although underreported in the literature, cases of symptomatic intra-articular HO after tibial nailing exist, and they synonymously describe associated anterior knee pain with radiographic findings of HO requiring excision.27–30 Tornetta et al and Krishnan et al both described HO involving the retropatellar space and infrapatellar fat pad.28,30 Our findings demonstrated that 37.5% of all cases with HO development were instances of retropatellar fat-pad calcification. In previously described cases, the observed interval between index surgery and symptomatic HO ranged from as little as 6 weeks28 up to 3.5 years.29 Interestingly, Howell et al reported on one patient with radiographic evidence of HO at the 4-week follow-up, which remained asymptomatic until 3.5 years later. This is consistent with HO presentation around the hip joint, which can become symptomatic rather quickly, yet while only radiographically apparent in other cases.7 Thus, to reliably capture the frequency of radiographic HO in this study, we determined a minimum of 6 weeks of follow-up as a reasonable observation period. Furthermore, our follow-up ranged from 6 weeks to 4.5 years and averaged 41 weeks, which would capture time points similar to that of prior literature, for patients who may become symptomatic later.

The pathophysiology of HO formation within the knee joint is currently unknown and poses the question as to whether similar biomolecular processes of dystrophic calcification described in HO of extra-articular areas lead to the observed radiographic HO in this cohort. This study observed a notable association of postoperative reaming debris with radiographic HO development, indicating the potential utility of thorough joint and/or surgical site irrigation as a protective measure, especially in the instance of ipsilateral retrograde femoral nailing. This observation also favors the notion of periarticular HO development possibly arising from displacement of bone matrix into the joint and/or surrounding soft tissues, an alternative pathway to previous reports describing HO after multiligamentous knee dislocations.12,23 However, similar to previous reports,22,23 we also identified correlating injury characteristics, including higher ISS, ICU admission, and chest injury. Our study did not find head injury to be associated with HO formation, which has been well-documented, in prior literature; however, this is likely because of this study being underpowered for this association. These relationships suggest a close correlation of HO development with the severity of local and systemic injury insult, which may advocate a heightened awareness during follow-up in this population.

We acknowledge several limitations to this study. First, inherent to its descriptive and retrospective study design, clinical outcome measures, differences in approach, and correlations should not be interpreted as causal. Rather, as the largest sample to date to evaluate the incidence of knee HO, this study's main findings are the observed incidence among reamed antegrade tibial IMN and associated injury characteristics. Given the event rate of 15% for HO in all-comer patients, the post hoc analysis was not adequately powered to detect a statistically significant difference in the clinical outcomes of knee pain and ROM. Therefore, future prospective investigations are encouraged to substantiate the clinical implications of our findings, as we did not find notable pain or motion-related problems in patients with radiographic evidence of HO. This study is the first to describe and characterize periarticular HO based on radiographic imaging. Furthermore, the correlation between retained postoperative retroinfrapatellar reaming debris and HO on last follow-up may question the specificity of the radiographic analysis, proposing that intra-articular bone seen on last radiograph could have been unchanged retained reaming debris. In those cases, HO was only charted when a notable progression in size, and radiolucency was appreciated.

The authors also acknowledge that the infrapatellar approach to tibial IMN may not necessarily violate the joint capsule, thus being at lower risk of intracapsular reaming debris. Whether postoperative retroinfrapatellar reaming debris in patients undergoing the infrapatellar approach were located extracapsular or intracapsular in the retroinfrapatellar region was not possible to reliably distinguish with plain radiographs. In addition, this was a retrospective data collection, with all the limitations inherent to chart review. In addition, patients who developed symptoms (knee pain and/or stiffness) may have been lost to follow-up as this study evaluated these variables at their last follow-up, with a range of 6 weeks to 4.5 years. Loss of follow-up would have likely biased toward a stronger association of knee pain as symptomatic patients should have had a stronger urgency to follow-up in clinic than patients without pain or limited ROM. In addition, this study is descriptive rather than clinical without a control group for comparison, and therefore, selection bias induced by loss of follow-up is less relevant for the design and purposes of this study. However, future studies should investigate the implications of HO on knee pain and stiffness as this study was not designed to comprehensively evaluate these parameters. Finally, this study did not take into account the possible presence of a genetic disposition increasing patients' susceptibility to HO.7 Nonetheless, in a comprehensive review, Meyers et al summarized that the most common pathogenesis of HO is predicated on a non-genetic pathway, with genetic forms only encountered rarely.

Conclusion

Our findings indicate that radiographic evidence of heterotopic bone in the knee joint after reamed IMN is not uncommon and is associated with retroinfrapatellar reaming debris, injury severity, ICU admission, and concomitant retrograde femoral nailing. No association was seen with surgical approach, which contradicts the hypothesis that the suprapatellar approach is a risk factor of retained reaming debris and HO. Additional prospective investigation with a larger sample size and comparative study design could help delineate clinical implications of HO after IMN.

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