Association of the Anterolateral Thigh Osteomyocutaneous Flap With Femur Structural Integrity and Assessment of Prophylactic Fixation | Facial Plastic Surgery | JAMA Otolaryngology–Head & Neck Surgery | JAMA Network
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Figure 1.  Femur Preparation
Femur Preparation

A, Sawbones (Pacific Research Laborator) fourth-generation composite femur with 10-cm, 30% circumferential osteotomy at the location of an anterolateral thigh osteomyocutaneous flap harvest. B, Osteotomized femur with intramedullary nail and distal interlock screw fixation. Image taken prior to second interlock screw placement and biomechanical testing.

Figure 2.  Biomechanical Testing Setup
Biomechanical Testing Setup

A, Femur prepared for anterior to posterior (AP) 4-point bend testing. B, Femur prepared for torsional testing. The lever arm extends from the metal base seating the proximal epiphysis.

Figure 3.  Four-Point Bend Testing Results
Four-Point Bend Testing Results

Four-point bend testing revealed significant decreases in (A) force to fracture and (B) stiffness following osteotomy. Error bars indicate 95% CIs. AP indicates anterior to posterior; PA, posterior to anterior.

Figure 4.  Torsional Testing Results
Torsional Testing Results

Torsional testing revealed significant decreases in (A) torque to fracture and (B) stiffness following osteotomy. Error bars indicate 95% CIs.

Figure 5.  Fractured Femurs Following Torsional Testing
Fractured Femurs Following Torsional Testing

Fractured femurs following torsional testing demonstrate identical spiral fractures through the distal osteotomy corner.

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Original Investigation
September 2018

Association of the Anterolateral Thigh Osteomyocutaneous Flap With Femur Structural Integrity and Assessment of Prophylactic Fixation

Author Affiliations
  • 1Department of Otolaryngology–Head and Neck Surgery, Medical University of South Carolina, Charleston
  • 2Department of Orthopaedic Surgery, Medical University of South Carolina, Charleston
  • 3Cancer Control Program, Hollings Cancer Center, Charleston, South Carolina
JAMA Otolaryngol Head Neck Surg. 2018;144(9):769-775. doi:10.1001/jamaoto.2018.1014
Key Points

Questions  Does anterolateral thigh osteomyocutaneous (ALTO) flap harvest cause changes in the structural integrity of the femur, and if so, are these changes mitigated by prophylactic fixation?

Findings  In this study of 30 synthetic composite femurs, there was a dramatic decrease in femur strength following ALTO harvest with a 10-cm-long and 30% circumferential osteotomy. While prophylactic fixation improved biomechanical strength, osteotomized femurs with fixation remained weaker than controls on both 4-point bend and torsional testing.

Meaning  Anterolateral thigh osteomyocutaneous flap harvest with a 10 cm–long and 30% circumferential osteotomy resulted in significant changes in the structural integrity of the femur that are not fully compensated by prophylactic fixation.

Abstract

Importance  The chimeric anterolateral thigh osteomyocutaneous (ALTO) free flap is a recently described microvascular option for head and neck osseous defects associated with complex soft-tissue requirements. To date, the association of ALTO flap harvest with femur structural integrity and the need for routine prophylactic fixation following harvest has been incompletely described.

Objective  To investigate the association of ALTO flap harvest, with and without prophylactic fixation, on femur structural integrity as measured by 4-point bend and torsional biomechanical testing.

Design and Setting  At a research laboratory, 24 synthetic fourth-generation composite femurs with validated biomechanical properties underwent 10-cm-long, 30% circumferential osteotomies at the proximal middle third of the femur; 6 femurs served as controls. Osteotomized femurs with and without fixation underwent torsional and 4-point bend biomechanical testing. Femur fixation consisted of intramedullary nail and distal interlock screw placement.

Main Outcomes and Measures  Force and torque to fracture (expressed in kilonewtons [kN] and Newton meters [N∙m], respectively) were compared between controls, osteotomized femurs without fixation, and osteotomized femurs with fixation. Additional outcome measures included femur stiffness and fracture patterns.

Results  On posterior to anterior (PA) 4-point bend testing, force to fracture of osteotomized femurs was 22% of controls (mean difference, 8.3 kN; 95% CI, 6.6-10.0 kN). On torsional testing the torque to fracture of osteotomized femurs was 12% of controls (mean difference, 351.1 N∙m; 95% CI, 307.1-395.1 N∙m). Following fixation there was a 67% improvement in PA force to fracture and a 37% improvement in torque to fracture. However, osteotomized femurs with fixation continued to have a reduced PA force to fracture at 37% of controls (mean difference, 6.8 kN; 95% CI, 4.5-9.2 kN) and torque to fracture at 16% of controls (mean difference, 333.7 N∙m; 95% CI, 306.8-360.6 N∙m). On torsional testing, all osteotomized femurs developed similar spiral fractures through a corner of the distal osteotomy site. This fracture pattern changed after prophylactic fixation with femurs developing nondisplaced fractures through the proximal osteotomy site. There were no underlying hardware failures during testing of osteotomized femurs with fixation.

Conclusions and Relevance  Anterolateral thigh osteomyocutaneous flap harvest results in significant changes in the structural integrity of the femur. Postoperative stabilization should be strongly considered, with future research directed at investigating the clinical significance of residual biomechanical changes following femur fixation.

Introduction

For head and neck surgeons, reconstruction of osseous defects with complex soft-tissue requirements remains a challenge. Some advocate for the use of flaps based on the subscapular system, while others prefer a 2-flap approach in this setting.1-4 Drawbacks of these options include challenges raising subscapular system flaps concurrently with tumor ablation and increased operative times.2,3

The chimeric anterolateral thigh osteomyocutaneous (ALTO) free flap is a recently described microvascular option for head and neck osseous defects associated with complex soft-tissue requirements.5,6 Advantages of the ALTO flap include its variable soft-tissue components and its ability to be raised concurrently with tumor ablation.7 During ALTO flap harvest, a corticocancellous segment of femur is removed along with its overlying vascularized periosteum. The bony portion of the flap is supplied by vessel branches to the vastus intermedius muscle, which are preserved and raised with the flap. With its versatile harvest options, the ALTO flap has been used for reconstruction of complex oral cavity and midface defects.5

Given the femur’s essential weight-bearing role, the effect of an osteotomy on its structural integrity is a necessary consideration. To date, published reports of the ALTO flap have not described instances of postoperative fracture.5,6 However, there are limited data on the biomechanical effects of ALTO flap harvest on the femur. In a study of uniaxial compression loading following 30% to 40% circumferential femoral osteotomies, Broderick et al8 did not find a change in load to fracture but did note a change in the fracture pattern of osteotomized femurs. However, this study did not assess other important biomechanical measures that could be effected by femur harvest, including 4-point bend and torsional strength. In addition, the potential role for prophylactic fixation of the osteotomized femur remains unknown. Prior research on the osteocutaneous radial forearm free flap (OCRFFF) has shown prophylactic stabilization to have a role in preventing postoperative fractures.9-12 Determining whether similar principles apply to the ALTO flap requires further study.

If the ALTO flap is to assume a role in the reconstruction of osseous head and neck defects, it is necessary to understand the donor site morbidity and potential benefit of prophylactic femur stabilization following harvest. In this study we investigate the biomechanical effect of a standardized femoral osteotomy with and without prophylactic fixation.

Methods
Femur Preparation

Thirty synthetic fourth-generation composite femurs with validated biomechanical properties (Sawbones, model 3406; Pacific Research Laboratories) were selected for testing. These femurs have been frequently used in orthopedic studies of femur fractures and are manufactured to simulate the femurs of men younger than 80 years with good bone quality.13-15

Osteotomies 10 cm in length and 30% in circumference were chosen based on the maximal ALTO harvest described by Brody et al.5 Twenty-four femurs were osteotomized at the location described for ALTO harvest using a reciprocating saw and custom cutting guide to facilitate a consistent osteotomy cut without angular variability (Figure 1A). The remaining 6 femurs were designated as controls.

Nine femurs underwent intramedullary (IM) nailing with distal interlock screw fixation prior to biomechanical testing. IM nailing was chosen because it is a commonly used technique in patients at risk for pathologic femur fractures from lytic lesions owing to metastatic cancer.16-19 The addition of distal interlock fixation to an IM nail has also been found to increase femur stability in the setting of a fracture.20,21 The femurs were instrumented using the Gamma3 IM nail system (Stryker) by a surgeon trained in this technique (T.M.P.). A lag screw was placed through the proximal portion of the IM nail in the center-center position of the femoral head. Two interlock screws were applied distally in the static position. Following hardware placement, radiographs were obtained to verify hardware position prior to testing (Figure 1B).

Study Design and Outcome Measures

Initial biomechanical testing was performed on the control femurs (n = 3 four-point bend tests, n = 3 torsional tests). Owing to the limited supply of femurs, we obtained control data in 1 four-point bending direction and choose an anterior to posterior (AP) bend because its strength and stiffness were similar to, or better than, other tested directions in prior studies.22,23 Eight osteotomized femurs without fixation underwent 4-point bend testing, and 7 underwent torsional testing. Because the osteotomy asymmetrically removed anterior femur cortex and could potentially cause directionally specific changes in 4-point bend strength, we performed both AP and posterior to anterior (PA) testing on the osteotomized femurs (n = 3 AP bend tests, n = 5 PA bend tests). After observing reductions in the force to fracture of osteotomized femurs on PA bend and torsional testing, the 9 femurs with rigid fixation were divided into 2 groups (n = 4 for PA bend tests, n = 5 for torsional tests) to evaluate the ability of fixation to mitigate these changes.

The peak axial force and torque achieved prior to fracture (force and torque to fracture) on 4-point bend and torsional testing was the primary outcome measure and has been previously used as a marker for bony strength.21,24 Other outcome measures included femur stiffness (a measure of the femur’s resistance to displacement) and fracture patterns.

Biomechanical Testing

Biomechanical testing was performed on an MTS 858 Mini Bionix II testing system. Data collected included displacement to fracture (in millimeters), and force to fracture (expressed in kilonewtons [kN]). For the 4-point bend testing, femurs were loaded into the testing apparatus, and a custom fiberglass mold was used to orient the femur in a physiologic position relative to the applied AP and PA forces (Figure 2A). The MTS device was set to have a constant axial displacement of 1 mm/s until femur fracture occurred. Torsional testing required the femurs to be fixed at each end using silicon and/or fiberglass molds contoured to the proximal and distal epiphyses (Figure 2B). The molds were seated within metal bases, and a lever arm was attached perpendicularly to the proximal base. The lever arm was set to have a constant displacement of 0.4 mm/s until femur fracture occurred.

Statistical Analysis

Statistical analysis was performed using SPSS software (version 24; IBM SPSS Inc). For the 4-point bend data, stiffness (kN/mm) was calculated using the slope of the linear portion of the force to fracture vs displacement curves. Torque (expressed in Newton meters [N∙m]) and angular displacement (degrees) were calculated for the torsional data and used to determine the torsional stiffness (N∙m/degrees) in a similar manner. Means and 95% CIs were determined, and independent sample t tests were used for comparisons between groups. All tests were 2-sided, with P < .05 considered significant.

Results
Osteotomized Femurs Without Fixation

Figure 3 summarizes the 4-point bend testing results. On AP loading, the force to fracture of osteotomized femurs was 84% of controls (mean difference [MD], 1.6 kN; 95% CI, −1.7 to 5.0 kN), and osteotomized femurs were 66% as stiff as controls (MD, 0.40 kN/mm; 95% CI, 0.18-0.62 kN/mm). PA testing revealed that osteotomized femurs had significantly decreased force to fracture at 22% of controls (MD, 8.3 kN; 95% CI, 6.6-10.0 kN), and stiffness at 61% of controls (MD, 0.46 kN/mm; 95% CI, 0.20-0.73 kN/mm).

The torsional testing results are summarized in Figure 4. Following osteotomy, femur torque to fracture was only 12% of controls (MD, 351.1 N∙m; 95% CI, 307.1-395.1 N∙m), and there was a similarly significant decrease in torsional stiffness at 9% of controls (MD, 19.0 N∙m/degree; 95% CI, 11.9-26.1 N∙m/degree).

Osteotomized Femurs With Fixation

Following fixation of the osteotomy with an IM nail and distal interlock screws, there was a 67% improvement in PA force to fracture (MD, 1.5 kN; 95% CI, 0.2-2.8 kN) and a 32% improvement in PA stiffness (MD, 0.23 kN/mm; 95% CI, −0.06 to 0.53 kN/mm) relative to osteotomized femurs without stabilization. However, relative to control femurs, osteotomized femurs with stabilization had significantly reduced PA force to fracture at 37% of controls (MD, 6.8 kN; 95% CI, 4.5-9.2 kN). Fixated femurs also continued to have decreased stiffness (82% of controls), although this difference was no longer statistically significant (MD, 0.23 kN/mm; 95% CI, −0.14 to 0.60 kN/mm).

Torsional testing of the fixated femurs revealed a 37% improvement in torque to fracture (MD, 17.4 N∙m; 95% CI, 5.9-29.0 N∙m), and a 324% improvement in torsional stiffness (MD, 6.2 N∙m/degree; 95% CI, 5.4-7.0 N∙m/degree) compared with osteotomized femurs without fixation. Despite these improvements, fixated femurs remained much weaker than controls with a torque to fracture of only 16% of controls (MD, 333.7 N∙m; 95% CI, 306.8-360.6 N∙m). Torsional stiffness also remained reduced at 39% of controls (MD, 12.8 N∙m/degree; 95% CI, 6.1-19.5 N∙m/degree).

Fracture Patterns

A reproducible fracture pattern was observed on torsional testing with all osteotomized femurs developing a spiral fracture through a corner of the distal osteotomy site (Figure 5). Following fixation, the torsional fracture pattern changed, and all fixated femurs developed a nondisplaced spiral fracture through a corner of the proximal osteotomy site.

On both 4-point bend and torsional testing, the IM nail and interlock screw fixation remained intact in all specimens. When fractures occurred in the osteotomized femurs with fixation, they were nondisplaced without an underlying hardware failure.

Discussion

The ALTO flap is a recently described microvascular option for osseous defects with complex soft-tissue requirements. Despite the femur’s important weight-bearing role, little is known about the effect of ALTO flap harvest on its structural integrity. We therefore undertook this study to investigate the biomechanical effect of a standardized femoral osteotomy and assess the role of prophylactic fixation in minimizing donor site morbidity.

Biomechanical Effect of Osteotomy

There are at least 3 factors that can lead to biomechanical changes after osteotomy of a long bone.24 An osteotomy decreases the amount of residual cortical bone available to share a given load, thereby increasing stress (force per unit area) on the remaining bone. Osteotomies also change the geometry of bones and can result in local stress concentration effects on bone immediately surrounding the osteotomy.25,26 Finally, the “open-section” effect refers to the reversal of shear flow along the inner cortex of a long bone under torsional stress, which can result in a greater than 50% reductions in torsional strength following osteotomies less than half the diameter of a bone in width.26

Following our standardized osteotomy, there were significant changes in force and torque to fracture as well as stiffness on 4-point bend and torsional testing. We observed a significant decrease in force to fracture with PA, but not AP 4-point bend testing. The directionally specific 4-point bend results likely reflect the reduced amount of anterior femur cortex available to distribute tension forces when bending in that direction. On torsional testing, we observed a greater than 85% reduction in the torque required to fracture the osteotomized femurs. In addition, there was a remarkable symmetry to the fracture patterns with all osteotomized femurs developing fractures through the distal osteotomy corners. The magnitude of the strength reduction and the reproducibility of the spiral fractures highlight the importance of local stress concentration effects along osteotomy corners during torsional stress.26

Biomechanical Effect of Prophylactic Fixation

Because osteotomized femurs were significantly weaker than controls, we investigated whether prophylactic fixation following osteotomy could restore femur structural integrity using a fixation technique frequently used to treat patients with lytic femoral lesions at high risk for fracture.16-18 Following fixation, we did see significant increases in the amount of force and torque required to cause a fracture relative to the osteotomized femurs without fixation. It was also notable that we did not observe any cases of fixation fracture or failure during biomechanical testing. The IM nail and interlock screws remained intact on all specimens, with the femurs developing nondisplaced fractures on both 4-point bend and torsional testing.

Despite the biomechanical benefit of IM nailing and interlock fixation, the osteotomized femurs with fixation remained significantly weaker than control femurs. This observation was not altogether surprising because a biomechanical study of radius fixation following OCRFFF harvest similarly found that while plating significantly improved 4-point bend and torsional strength, the fixated radius strength ratio remained only 64% to 70% that of controls.11 We were surprised, though, by the strength disparity between fixated femurs and controls because IM nailing has been successfully used in clinical practice for reconstruction of segmental femur defects more extensive than those in our osteotomy.27,28 Unfortunately, previous biomechanical studies of lytic lesions, as well as traumatic femur fractures, lack either control data on intact femurs or data on the residual weakness of fixated femurs compared with intact femurs.13,15,20,21,29,30 Accordingly, we were unable to identify a comparison study reporting how much weaker a successfully fixated femur is expected to be than an intact femur in these settings.

Implications for Patient Care

After the initial development of the OCRFFF, early reports did not describe a need for prophylactic plating of the distal radius following flap harvest.31,32 Biomechanical studies were later performed, demonstrating dramatic reductions in radius strength with a mean postoperative fracture rate of 24% observed across several case series.9,24,33 Similarly, the significant decreases in force to fracture we observed following simulated ALTO flap harvest raise concerns regarding the risk for postoperative femur fractures. While Brody et al5 did not report any fractures in in their case series, most patients had less than 1 year of follow-up. In the absence of a large series of patients with long-term clinical follow-up, biomechanical studies such as ours can provide insight into the postoperative fracture risk. Although there are limited quantitative data on the extent of femur weakening necessary to place patients at increased risk for fracture, one biomechanical study29 found that a clinically “high-risk” femoral neck lesion (recommended to receive prophylactic fixation) resulted in a 48% reduction in axial compression load to failure. Although we could not identify a comparable study of a diaphyseal lesion at the location for ALTO flap harvest, the greater than 50% reduction in 4-point bend and torsional strength we observed in osteotomized femurs without fixation raises important concerns regarding postoperative fracture risk. Therefore, we would recommend prophylactic stabilization of the femur following ALTO flap harvest and have made it the routine practice at our institution.

The extent to which prophylactic fixation will mitigate the risk of postoperative fracture following ALTO flap harvest remains unknown. In our study, osteotomized femurs with fixation achieved a 37% PA bend force to fracture of controls and a 16% torque to fracture of controls. Whether these significant biomechanical differences in strength result in a clinically elevated fracture risk requires further study. There is precedent from the OCRFFF literature that prophylactic stabilization, while not returning postharvest radius strength to baseline, is sufficient to drastically decrease the rate of postoperative fractures. Radius strength following OCRFFF harvest with fixation is approximately 30% to 36% decreased relative to native radii, but clinically, this manifests in a symptomatic fracture rate of 0% to 2%.9-12,34,35 There are also robust outcomes data demonstrating the ability of an IM nail to successfully treat impending or pathologic femur fractures owing to lytic lesions. Across multiple studies encompassing more than 300 patients with impending or pathologic femur fractures due to lytic lesions, there was less than a 2.5% rate of IM nail failure resulting in fracture or biomechanical instability in follow-up.16-19 In extrapolating these data to head and neck patients undergoing ALTO flap harvest with prophylactic fixation, there are important caveats. Lytic lesions in these studies varied in size, location, and number (single vs multiple) and therefore may not have caused the same degree of biomechanical weakening as ALTO flap harvest. Patients with lytic lesions also have limited survival owing to their metastatic cancer. Thus, these data do not address the ability of an IM nail to prevent a fracture at an osteotomy site in long-term follow-up. The extent to which bony remodeling at the ALTO flap harvest site may restore femur biomechanical strength and limit the need for fixation in the years following surgery is also unknown. However, if a femur fracture were to occur, our study demonstrated the role of prophylactic fixation in preventing fracture displacement. The IM nail and interlock screws remained intact during all biomechanical tests, suggesting that in clinical practice, femur fractures occurring in the setting of fixation would be controlled and could likely be treated nonoperatively with a change in weight-bearing status.

Limitations

This study has several important limitations. First, the study was conducted using synthetic femurs designed to replicate the femurs of middle-aged men with good bone quality. Whether our data accurately replicate the in vivo biomechanical effects of ALTO flap harvest in elderly and female patients with osteopenia is therefore unknown.36 Second, no data exist to estimate the risk of femur fracture given a specific change in biomechanical strength as we measured following simulated ALTO flap harvest. Therefore, our biomechanical data cannot be used to determine the absolute fracture risk, need for limited weight-bearing status, or need for activity level restrictions following ALTO flap harvest. Third, we only tested 1 osteotomy and chose the largest size reported by Brody et al5 to provide data regarding the upper limit of what is being clinically performed. However, previous studies have suggested that osteotomies less than 20% in circumference and less than 2 outer diameters in length cause smaller reductions in torsional stiffness and strength.26,37 Given the diameter of the typical adult femur, this implies that osteotomies less than 20% in circumference and less than 5 cm in length may have significantly smaller biomechanical effects than our study osteotomy, which may alter the need for postoperative fixation. Fourth, we performed square-end osteotomies, but prior research in the OCRFFF literature has suggested that keel-shaped or beveled osteotomies have improved torsional strength compared with square-end osteotomies.24,38 Future studies of the ALTO flap should therefore investigate the effect of varying the osteotomy length, circumference, shape, and fixation technique on femur strength using models that replicate the in vivo biomechanics and bone quality of male and female patients across the lifespan. Additional studies are also needed to better address translating these biomechanical investigations into clinically meaningful patient care recommendations.

Conclusions

Following ALTO harvest there are significant changes in the biomechanical properties of the femur; therefore, postoperative fixation should be strongly considered. Future studies should investigate the clinical significance of residual biomechanical differences following femur fixation as well as the effect of varying the osteotomy and femur bone quality.

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Article Information

Corresponding Author: Mitchell L. Worley, MD, Department of Otolaryngology–Head and Neck Surgery, Medical University of South Carolina, 135 Rutledge Ave, MSC 550, Charleston, SC 29425 (worleym@musc.edu).

Accepted for Publication: April 5, 2018.

Published Online: July 26, 2018. doi:10.1001/jamaoto.2018.1014

Author Contributions: Drs Worley and Patterson had full access to all the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.

Study concept and design: Worley, Graboyes, Wu, Hornig, Walton.

Acquisition, analysis, or interpretation of data: Worley, Patterson, Graboyes, Wu, Brody, Walton.

Drafting of the manuscript: Worley, Patterson, Graboyes.

Critical revision of the manuscript for important intellectual content: Patterson, Graboyes, Wu, Brody, Hornig, Walton.

Statistical analysis: Worley, Patterson, Walton.

Obtained funding: Graboyes, Hornig, Walton.

Administrative, technical, or material support: Patterson, Graboyes, Wu, Brody, Walton.

Study supervision: Graboyes, Wu, Brody, Hornig, Walton.

Conflict of Interest Disclosures: All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Dr Worley received 2 free textbooks from the Depuy Synthes Resident Book Program. No other conflicts are reported.

Funding/Support: Drs Graboyes, Hornig, and Walton received funding for this project through National Institutes of Health/National Center for Advancing Translational Sciences grant No. UL1TR001450.

Role of the Funder/Sponsor: The National Institutes of Health had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.

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