Redefining the nasal septal L-strut: A quantitative analysis of septal arcs and angles.

OBJECTIVE: To define and quantify common points and relationships within the anterior nasal septum and recognize the anatomic curvature of the dorsal and caudal septum as arcs with certain rises. STUDY DESIGN: Case series study. METHODS: Forty cadaveric quadrangular cartilages were dissected. Points along the septum were defined and relationships measured. Angles that composed the anterior septum using values that did account for the curvature of the anterior nasal septum, or classical L-strut values, were compared to C-strut values, which included rises away from the L-strut. RESULTS: The mean arc of the nasal septum dorsum was significantly longer than the caudal arc (25.9 ± 5.14 mm vs. 23.96 ± 4.07 mm, P = 0.048); the mean rise of the dorsal arc was significantly shorter than the rise of the caudal arc (3.04 ± 1.25 mm vs. 3.88 ± 1.42 mm, P = 0.006). The mean point of furthest rise of the dorsal arc was slightly anterior to the midpoint of the dorsal strut (13.43 ± 5.07 mm); the mean caudal equivalent was slightly posterior to the midpoint of the caudal strut (8.55 ± 4.33 mm). Septal angles measured using L-strut versus C-strut values were significantly different. CONCLUSION: The L-strut leading edge is not congruent with the anatomic septum and can be better described as a C-strut with arcs and rises away from classically interpreted straight L-strut lines, all of which may guide surgeons in the creation of a more natural, supportive cartilage framework while encouraging quantifiable septorhinoplasty teaching. LEVEL OF EVIDENCE: NA. Laryngoscope, 1806-1810, 2018.

Yield Strength Testing in Human Cadaver Nasal Septal Cartilage and L-Strut Constructs.

IMPORTANCE: To our knowledge, yield strength testing in human nasal septal cartilage has not been reported to date. An understanding of the basic mechanics of the nasal septum may help surgeons decide how much of an L-strut to preserve and how much grafting is needed. OBJECTIVES: To determine the factors correlated with yield strength of the cartilaginous nasal septum and to explore the association between L-strut width and thickness in determining yield strength. DESIGN, SETTING, AND PARTICIPANTS: In an anatomy laboratory, yield strength of rectangular pieces of fresh cadaver nasal septal cartilage was measured, and regression was performed to identify the factors correlated with yield strength. To measure yield strength in L-shaped models, 4 bonded paper L-struts models were constructed for every possible combination of the width and thickness, for a total of 240 models. Mathematical modeling using the resultant data with trend lines and surface fitting was performed to quantify the associations among L-strut width, thickness, and yield strength. The study dates were November 1, 2015, to April 1, 2016. MAIN OUTCOMES AND MEASURES: The factors correlated with nasal cartilage yield strength and the associations among L-strut width, thickness, and yield strength in L-shaped models. RESULTS: Among 95 cartilage pieces from 12 human cadavers (mean [SD] age, 67.7 [12.6] years) and 240 constructed L-strut models, L-strut thickness was the only factor correlated with nasal septal cartilage yield strength (coefficient for thickness, 5.54; 95% CI, 4.08-7.00; P < .001), with an adjusted R2 correlation coefficient of 0.37. The mean (SD) yield strength R2 varied with L-strut thickness exponentially (0.93 [0.06]) for set widths, and it varied with L-strut width linearly (0.82 [0.11]) or logarithmically (0.85 [0.17]) for set thicknesses. A 3-dimensional surface model of yield strength with L-strut width and thickness as variables was created using a 2-dimensional gaussian function (adjusted R2 = 0.94). Estimated yield strengths were generated from the model to allow determination of the desired yield strength with different permutations of L-strut width and thickness. CONCLUSIONS AND RELEVANCE: In this study of human cadaver nasal septal cartilage, L-strut thickness was significantly associated with yield strength. In a bonded paper L-strut model, L-strut thickness had a more important role in determining yield strength than L-strut width. Surgeons should consider the thickness of potential L-struts when determining the amount of cartilaginous septum to harvest and graft. LEVEL OF EVIDENCE: NA.

A Model to Estimate L-Strut Strength With an Emphasis on Thickness.

IMPORTANCE: To perform and teach septorhinoplasty, one must have a principled understanding of the mechanics of the nasal septum. The thickness of the L-strut and how it changes septal strength have not been adequately quantified, yet calculating septal strength based on changes to thickness and size is vital in maintaining lasting nasal strength and integrity. OBJECTIVE: To establish standards for the nasal septal cartilage thickness, dorsal and caudal septum length, and Young's modulus. To provide a basis for quantitative, operative decision making, a mathematical model of L-strut strength is presented based on changes in thickness and width. DESIGN, SETTING, AND PARTICIPANTS: Nasal septal cartilages from 30 fresh cadavers were used to measure thickness at clinically relevant points and length of dorsal and caudal L-strut arms. The Young modulus was directly measured using a force gauge. Statistical analyses were performed to compare thicknesses in anatomically relevant areas. Using a cantilevered beam construct, the spring constant of the L-strut dorsal and caudal arms were estimated individually with width and thickness as variables. MAIN OUTCOMES AND MEASURES: Thickness, dorsal and caudal length, and the Young modulus of nasal septal cartilage. Spring constants of dorsal and caudal L-strut arms with different combinations of thickness and width. RESULTS: The mean (SD) age at death of the 30 cadavers was 79.2 (13.6) years (range 50-97 years). Of these, 17 (57%) were male, and 13 (43%) were female. The mean (SD) nasal septal cartilage thickness in the 30 cadavers was 1.45 (0.54) mm. Mean (SD) thickness of points along the 2-mm L-strut line was 1.49 (0.56) mm and was significantly thicker than points along the 5-mm L-strut line (mean [SD] thickness, 1.29 [0.52] mm) but significantly thinner than points along the 15-mm L-strut line (mean [SD] thickness, 1.68 [0.53]). Mean (SD) thicknesses of the posterior dorsal and caudal cartilage points were 1.52 (0.45) mm and 1.71 (0.69) mm and were significantly thicker than the anterior dorsal and caudal points (mean [SD] thickness, 1.28 [0.42] mm and 1.31 [0.44] mm, respectively). Mean (SD) dorsal and caudal L-strut arm lengths were 21.9 (3.7) mm and 20.9 (3.5) mm, respectively. The mean (SD) Young modulus was 2.03 (1.3) MPa. A model was generated demonstrating the thickness required to maintain a desired strength at a given dorsal or caudal arm width. CONCLUSIONS AND RELEVANCE: Although thickness was not uniform throughout the nasal septum, there is a predictable pattern. Thickness of the L-strut contributes more to septal strength than does L-strut width. The model generated in this study can be used in planning, performing, or teaching the applied mechanics of septorhinoplasty. LEVEL OF EVIDENCE: NA.