Association Between the Thickness, Width, Initial Curvature, and Graft Origin of Costal Cartilage and Its Warping Characteristics

Abstract

This study compares the degree of warping in costal cartilage specimens having different thicknesses, widths, initial curvatures, and origin relative to the central cross-section.

Costal cartilage is an important graft source for secondary rhinoplasty and nasal reconstruction, especially when other autologous sources are exhausted or insufficient. However, understanding the warping characteristics of costal cartilage is critical to achieving more predictable postoperative results.¹ This study compares the degree of warping in costal cartilage specimens having different thicknesses, widths, initial curvatures, and origin relative to the central cross-section.

Methods

This study was conducted from July 1 to September 23, 2018. In accordance with Institutional Animal Care and Use Committee guidelines, porcine rib was obtained from a local packing house. The perichondrium was removed from the third and fourth ribs and stored in phosphate-buffered saline at ambient temperature. A cartilage guillotine was then used to obtain peripheral cartilage slices having 1-, 2-, and 3-mm thicknesses.² Similar slices were obtained from both the concave and convex sides of each rib, and then trimmed to a length of 4 cm and a width of 1 cm (Figure). Additional samples having a 2-mm thickness and 4-cm length, but half the width (0.5 cm), were also obtained. A total of 15 samples were used per each comparison group. Based on the University of California–Irvine Institutional Review Board requirements for nonhuman participant research determination, the present study did not require institutional review board approval.

Figure. Costal Cartilage Slices Obtained From the Concave and Convex Sides of the Rib.

The slices are obtained 1 mm from the periphery.

Photographs were taken of the specimens at 0, 0.5, 1, 5, and 24 hours after carving using a digital single-lens reflex camera (EOS Rebel T5i, Canon Inc) in a fixed position with the lens perpendicular to a guide that provided consistent alignment of the grafts. The degree of warping was calculated using the previously described arctangent method.³

Statistical significance between group means was determined by a 1-way t test and results were deemed statistically significant at P < .05. The effect of the initial curvature of costal grafts on overall warping after 24 hours was analyzed using linear regression.

Results

All cartilage slices warped during the entire 24-hour evaluation period, with most warping occurring early within the first hour after carving. Overall, thinner and narrower specimens warped more than thicker and wider specimens. For all thicknesses, there was a positive correlation between the degree of initial curvature (t = 0 minutes) and amount of warping 24 hours after carving. The correlation was increased with thinner specimens. When the rib grafts were initially sliced (t = 0 minutes), significantly increased curvature was noted in slices from the concave rib surface compared with those from the convex rib surface for all thicknesses. This trend continued 24 hours after carving (Table).

Table. Association Between Graft Characteristics and Warping.

Graft Thickness	Amount Warping From 0 to 30 min	Amount Warping From 0 to 24 h
Association of Graft Thicknesses With Warping Over Time
Comparison of thicknesses
1 mm vs 2 mm	8.8° vs 6.5°a	10.2° vs 7.4°b
1 mm vs 3 mm	8.8° vs 6.4°c	10.2° vs 7.0°d
2 mm vs 3 mm	6.5° vs 6.4°	7.4° vs 7.0°
Association of Graft Widths With Warping Over Time
Comparison of widths
0.5 cm vs 1 cm	9.7° vs 6.5°b	NA
Association of Slice Origin With Warping Over Time
Thickness, mm	0 min (concave vs convex)	24 h (concave vs convex)
1	7.1° vs 2.8°e	10.2° vs 4.2°e
2	5.5° vs −2.4°e	7.4° vs −1.8°e
3	5.5° vs −4.4°e	7.0° vs −5.2°e
Association of Initial Curvature With Overall Warping After 24 h
Graft thickness, mm	Slopef	R2
1	0.63	0.61
2	0.28	0.63
3	0.13	0.62

Abbreviation: NA, not applicable.

^aP = .047.

^bP = .01.

^cP = .02.

^dP = .004.

^eP < .001.

^fSlope defined as amount of warping over 24 hours to initial curvature.

Discussion

We identified a trend of decreased warping in cartilage slices of increased thickness; however, as thickness increased, this trend became less statistically significant. This finding falls in line with previous studies demonstrating that grafts greater than 4 mm did not result in significant warping regardless of central or peripheral origin.^4,5 This finding suggests that, at some point, a cartilage thickness will be reached where graft curvature will not be clinically relevant.

Our investigation presents a novel consideration of convex vs concave graft origin. Over time, grafts obtained from the convex surface of the rib remained flatter (less curvature) compared with grafts obtained from the concave rib surface. This difference is reasonable, considering that cartilage slices obtained from the convex rib surface started with curvature away from the perichondrium. Over time, the cartilage warps and curves in the opposite direction toward the perichondrium, as described by Gillies.⁶ Because the initial curvature and direction of warping are in different directions, the final shape is straighter.

We also demonstrated that grafts starting with a high degree of curvature are associated with increased warping over time. Ideally, such grafts with a high degree of curvature should be avoided unless the curvature is exploited or advantageously positioned so that continued warping would be in a more favorable direction.

Graft dimensions are often limited in nasal reconstruction surgical procedures to maintain aesthetics and avoid narrowing of the airway. However, an understanding of warping characteristics allows for more efficient and predictable use of autologous costal cartilage.