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Original Investigation |

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

Nicholas Paul, BS1; Kelton Messinger, BS1; Yuan F. Liu, MD2; Daniel I. Kwon, MD2; Cherine H. Kim, MD, PhD2; Jared C. Inman, MD2
[+] Author Affiliations
1Loma Linda University School of Medicine, Loma Linda, California
2Department of Otolaryngology–Head and Neck Surgery, Loma Linda University Medical Center, Loma Linda, California
JAMA Facial Plast Surg. 2016;18(4):269-276. doi:10.1001/jamafacial.2016.0136.
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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.

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Figure 1.
Thickness, Length, and Width of the L-Strut
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Figure 2.
Points of Thickness Measurement as Labeled by Dots

Mean thickness (in millimeters) and percentages relative to overall mean thickness of 1.45 mm are labeled for all points. Rows of 4 points each from top to bottom indicate posterior dorsal, anterior dorsal, anterior caudal, posterior caudal regions. ASA indicates anterior septal angle; PSA, posterior septal angle.

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Figure 3.
Spring Constant, k, for Various L-Strut Dorsal or Caudal Arm Thicknesses at Given Widths for a 21.9-mm-Length Beam

Lines are labeled for widths (mm), and odds only from 9 to 15 mm. The dashed line represents the spring constant of a 1.45-mm-thick dorsal or caudal arm with a width of 15 mm (2.209 N/m).

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