A Comparison of Costal Cartilage Warping Using Oblique Split vs Concentric Carving Methods

Key Points

Question

Does the oblique split method of costal cartilage carving in cadaveric specimens result in less warp than the concentric carving method?

Findings

This in vitro cadaveric study found no significant difference in the amount of warp between the oblique split and concentric carving methods during a 3-month period.

Meaning

The oblique split method of carving is a new, simpler method of harvesting costal cartilage for reconstructive rhinoplasty that is equivalent to the current criterion standard technique.

Abstract

Importance

Warping of costal cartilage is well described; however, its strength and abundance still make it a desirable graft material, especially in complex reconstructive rhinoplasty. Despite multiple methods of cartilage harvest, carving, and preimplantation treatment being developed over the years, warp remains a challenging clinical problem.

Objective

To assess whether the 30° oblique split method of preparing costal cartilage grafts produces less warping of the graft than the current standard of harvesting the central segment of a rib using the concentric carving method.

Design, Setting, and Participants

This in vitro cadaveric study evaluated the warping of costal cartilage grafts using the oblique split method with an angle of 30° or the concentric carving method during a 3-month period beginning in December 2014.

Main Outcomes and Measures

Millimeters of warp from baseline (at 1 hour) and at 1, 2, and 3 months, measured in the frontal and lateral planes.

Results

Among 74 costal cartilage grafts (using the oblique split method with an angle of 30° in 41 and the concentric carving method in 33), the mean amount of warp in the frontal plane was between 1.12 mm (95% CI, 0.96-1.28 mm) and 1.57 mm (95% CI, 0.94-2.20 mm) for the oblique group and between 1.18 mm (95% CI, 0.98-1.38 mm) and 1.29 mm (95% CI, 0.86-1.72 mm) for the concentric group during the 3-month period. There was no statistically significant difference in the frontal plane between the 2 methods at 1 hour (P = .45; 0.10 mm, 95% CI, −0.38 to 0.17 mm), 1 month (P = .32; 0.13 mm, 95% CI, −0.13 to 0.40 mm), 2 months (P = .50; 0.28 mm, 95% CI, −0.55 to 1.11 mm), or 3 months (P = .15; 0.22 mm, 95% CI, −0.08 to 0.52 mm) using the t test, regression analysis, or panel data analysis. Similarly, no significant difference was found in the lateral plane at 1 hour (P = .89; 0.04 mm, 95% CI, −0.49 to 0.56 mm), 1 month (P = .82; 0.07 mm, 95% CI, −0.56 to 0.70 mm), 2 months (P = .29; 0.40 mm, 95% CI, −0.36 to 1.17 mm), or 3 months (P = .63; 0.22 mm, 95% CI, −0.70 to 1.13 mm) using the t test. Two grafts were excluded due to desiccation.

Conclusions and Relevance

The 30° oblique split and concentric carving methods of costal cartilage graft carving are equivalent in terms of the amount of warp. The oblique split method may be superior because of easier carving and the increased volume of material.

Level of Evidence

NA.

This in vitro cadaveric study assesses whether the 30-degree oblique split method of preparing costal cartilage grafts produces less warping of the graft than the current standard of harvesting the central segment of a rib using the concentric carving method.

Introduction

It has long been appreciated that costal cartilage provides a good source of donor material in primary rhinoplasty, where structural support is required, and in revision cases, where septal and conchal cartilage supplies have been depleted. However, surgeons are sometimes reluctant to use the material because of perceived increased morbidity and its inherent tendency to warp.

Since Gillies first reported the phenomenon of cartilage warping in 1920, many techniques have been used to mitigate this limitation. His suggestion of stripping the perichondrium was subsequently proved inadequate, as was delayed implantation of the graft and using oppositional sutures. Other techniques investigated include “killing” the cartilage by boiling or irradiation or adding structural support by way of Kirschner wires or lamination or combining the cartilage with bone. While these methods were reportedly successful at reducing the amount of warp, some also had drawbacks: dead cartilage will inevitably resorb, and the addition of Kirschner wires introduced new complications, such as wire extrusion and infection.

Gibson and Davis first reported on the principle of the balanced section, cutting the cartilage in such a way that the intrinsic oppositional forces are in equilibrium, causing them to be neutralized. Since then, many other authors have tried to refine this principle to further reduce the amount of warp or the operative morbidity. Studies have also been performed to evaluate whether the level of rib, orientation of the balanced section, or storage conditions of the graft affect warpage: none had a significant effect.

A notable new technique was described in 2013 by Taştan et al. This procedure still endorsed the principle of the balanced section advocated by Gibson and Davis, but instead of carving the graft in the line of the long axis of the rib, the grafts are obtained at an oblique angle to the long axis (usually 30° or 45°). As shown in Figure 1, many battens can be created from a single piece of rib, resulting in minimal wastage. This procedure is in contrast to previous techniques in which most of the rib is discarded. In a study of 43 patients using autogenous costal cartilage grafts carved by the oblique split method, there was no incidence of clinically apparent warping, and it was confirmed that the grafts remained straight in 2 cases of reoperation for other causes.

Figure 1. Technique for the Preparation and Measurement of Rib Grafts.

Many battens can be created from a single piece of rib, resulting in minimal wastage.

There does not seem to be a consensus regarding when warp occurs in costal cartilage grafts. Gibson and Davis originally reported that, in vivo, the curvature of a graft did not seem to increase over time from the point at which it was first noticed to have warped. Thinner grafts were associated with faster time to warp in laboratory experiments; however, warp was apparent in all samples at 24 hours and did not increase after 48 hours. Harris et al seemed to confirm this finding, reporting that 90% of maximum warp occurred within 15 minutes of harvesting a centrally cut graft. In contrast, other studies have shown continued warping during a longer period of 2 to 4 weeks.

The objective of our study was to assess whether the 30° oblique split method of preparing costal cartilage grafts produces superior results with respect to warping compared with the current standard of harvesting the central segment of a rib in a concentric fashion. Millimeters of warp from baseline (at 1 hour) and at 1, 2, and 3 months were measured in the frontal and lateral planes.

Methods

The technique for the preparation and measurement of the grafts is shown in Figure 1. The UK National Health Service Research Ethics Committee review tool was used, and the dissection laboratory of The University of Nottingham confirmed that ethical approval was not required for this study.

Preparation of the Grafts

Four fresh-frozen human cadavers were used with permission of The University of Nottingham. Three were male, and one was female, with a mean age of 82 years (age range, 67-94 years). Costal cartilage of ribs 2 through 10 was obtained bilaterally from each of the cadavers. If we were unable to cut through the cartilage with a dermatome blade, the cartilage was considered too heavily ossified and was discarded. Thirty-four cartilaginous portions of ribs were found to be suitable from which to harvest grafts, spread evenly across 3 of the cadavers, with a smaller contribution from cadaver 1. Each costal cartilage was denuded of the perichondrium, wrapped in saline-soaked gauze, and stored in a labeled, sealable plastic bag. The samples were allocated to the oblique split group or the concentric carving group, ensuring that the groups were balanced according to the level of rib and side from which costal cartilage was harvested. We had originally allocated an equal number of ribs from each cadaver to the oblique split and concentric carving groups, but we found that each rib carved obliquely produced many more samples than those carved concentrically. Therefore, we elected to adjust the harvesting plan as listed in Table 1 to ensure that we had similar numbers in each group and enough concentric carving samples to ensure statistical comparison, although this adjustment imbalanced the allocation from each cadaver. In both groups, battens of 4 × 2 × 30 mm were fashioned using a combination of a dermatome blade, a scalpel, and a Gubisch cartilage measuring block to act as a template, allowing easy replication of the batten size. This method simulates a clinically useful graft for rhinoplasty fashioned in an appropriate manner.

Table 1. Harvesting Plan Demonstrating Allocation of Donor Rib to the Oblique Split and Concentric Carving Grafts, Including Change of Allocation of 3 Ribs to Balance the Number of Samples in Each Group.

Variable	Cadaver 1		Cadaver 2		Cadaver 3		Cadaver 4
	Left	Right	Left	Right	Left	Right	Left	Right
Rib 2	NA	NA	NA	NA	NA	NA	NA	Oblique
Rib 3	NA	NA	NA	Oblique	Oblique→concentric	NA	Oblique	Concentric
Rib 4	NA	NA	NA	Concentric	NA	Oblique→concentric	NA	Oblique
Rib 5	NA	NA	Concentric	Oblique	Oblique→concentric	Concentric	Oblique	Concentric
Rib 6	NA	NA	Oblique	Concentric	Concentric	Oblique	Concentric	Oblique
Rib 7	Concentric	Oblique	Concentric	Oblique	Oblique	Concentric	Oblique	Concentric
Rib 8	NA	Concentric	Oblique	NA	Concentric	Oblique	Concentric	Oblique
Rib 9	NA	NA	NA	NA	NA	NA	NA	NA
Rib 10	NA	NA	NA	NA	NA	NA	NA	NA

Abbreviation: NA, not applicable.

The concentric battens were carved in the anteroposterior orientation, and the oblique battens were cut at 30° to the longitudinal axis, as measured using a protractor. All battens were carved and measured within 1 hour of perichondrial stripping. All carving was performed by one of us (C.F.), an experienced facial plastic surgeon consultant, and all measuring throughout the study duration was performed by another one of us (G.W.), who was masked as to which method of carving had been used for each costal cartilage.

Measurement of the Grafts

Seventy-four costal cartilage grafts were created from the 34 rib segments: 41 were carved according to the oblique split method, and 33 were carved according to the concentric carving method. Each batten was given a unique identifier, and a 10-point grid was marked on the 4 × 30-mm (frontal) surface with a fine-tipped nail varnish pen, together with a dot that approximated the center point. Each end was color coded to ensure that the orientation was not lost over time. Three further points were also marked at equal points along the 2 × 30-mm (lateral) surface. A schematic drawing was left by the measuring apparatus labeling the dots from 0 to 10 and A, X, and B to ensure that the notation of the measurements always occurred in the same order among different grafts and over time. Unlike in previous studies in which indirect measurement of warp was calculated by retrospectively analyzing photographs, we opted for a direct measurement technique. A surface table was placed on a laboratory bench to ensure a completely flat and reproducible surface. A digital height gauge with a measuring accuracy to 0.01 mm was set up on the surface table, and a dial test indicator of the same accuracy was mounted on the measuring arm. A tripod with a flat top was set up next to the digital height gauge on the surface table and leveled. The costal cartilage sample was then placed on top of the tripod, with the frontal surface marked with the dots from 0 to 10 facing uppermost. The dial test indicator was placed on the central dot (dot 0), adjusting the height of the gauge until the smallest perceived flicker of the dial test indicator needle. This flicker indicated that contact was made while applying minimal pressure to the cartilage. The digital height gauge was reset to 0, creating a reference point (the datum) for the remaining dots. The gauge was then adjusted so that contact was made with each of the points 1 to 10 (again using the dial test indicator rather than the naked eye), and the value on the digital height gauge at each point was noted in a table. In the vertical plane, dots higher than the reference point 0 had a positive number, and dots lower than the datum had a negative number. The cartilage was then rotated through 90° so that the lateral surface with points A, X, and B was uppermost, and the process was repeated using X as the datum. This repetition enabled a 3-dimensional representation of the cartilage graft, evaluating warp in 2 planes (frontal and lateral). The process was repeated for all of the costal cartilage samples across the following 4 time points: within 1 hour of carving (December 2014) and 1, 2, and 3 months after carving. Between time points, the costal cartilage samples were rewrapped in saline-soaked gauze, placed inside a sealed plastic bag, put in an airtight plastic container, and left at room temperature. These storage conditions are similar to those described in other studies of warp.

Calculation of Warp

Warp can be calculated by measuring the difference between the largest distance above and below the reference plane. The difference between the highest and lowest value was calculated for each rib at each time point in both the frontal and lateral planes, called the frontal and lateral warp values, respectively. A t test was then performed on the warp values to assess if there was a difference between the oblique split and concentric carving groups at each time point and whether warp increased over time in either group. The results were then double-checked using a regression analysis and a panel data analysis.

Results

Frontal Plane

As shown in Figure 2, there was no statistically significant difference in the frontal plane between the oblique split and concentric carving samples at 1 hour (P = .45; 0.10 mm, 95% CI, −0.38 to 0.17 mm), 1 month (P = .32; 0.13 mm, 95% CI, −0.13 to 0.40 mm), 2 months (P = .50; 0.28 mm, 95% CI, −0.55 to 1.11 mm), or 3 months (P = .15; 0.22 mm, 95% CI, −0.08 to 0.52 mm) using the t test. The mean warp values for the 2 methods over time are listed in Table 2.

Figure 2. Mean Warp in the Frontal Plane Over Time for Oblique Split and Concentric Carving Grafts.

Differences between the 2 groups are shown at 4 time points.

Table 2. Mean Warp Over Time for Oblique Split and Concentric Carving Grafts^a.

Variable	1 h	1 mo	2 mo	3 mo
Oblique, mm	1.19 (1.02-1.36)	1.12 (0.96-1.28)	1.57 (0.94-2.20)	1.40 (1.20-1.60)
Concentric, mm	1.29 (1.07-1.51)	1.25 (1.04-1.46)	1.29 (0.86-1.72)	1.18 (0.98-1.38)
Difference, mm	0.10 (−0.38-0.17)	0.13 (−0.13-0.40)	0.28 (−0.55-1.11)	0.22 (−0.08-0.5)

^aThe mean difference between the 2 groups across the 4 time points was 0.18 (95% CI, 0.05-0.31).

The differences in measured warp value between the oblique split and concentric carving groups across the time points are also listed in Table 2. The mean difference in warp between the 2 groups across the 4 time points was 0.18 mm (95% CI, 0.05 to 0.31 mm).

Regression analysis demonstrated no significant change in the oblique split group during the study period (P = .27; 0.52 mm, 95% CI, −0.41 to 1.44 mm). Panel data analysis also showed no significant change during the 3-month period (P = .48; 0.19 mm, 95% CI, −0.33 to 0.71 mm) or between the 2 groups at each time point (P = .78; 0.04 mm, 95% CI, −0.26 to 0.34 mm).

Lateral Plane

As shown in Figure 3, there was no statistically significant difference in the lateral plane between the oblique split and concentric carving samples at 1 hour (P = .89; 0.04 mm, 95% CI, −0.49 to 0.56 mm), 1 month (P = .82; 0.07 mm, 95% CI, −0.56 to 0.70 mm), 2 months (P = .29; 0.40 mm, 95% CI, −0.36 to 1.17 mm), or 3 months (P = .63; 0.22 mm, 95% CI, −0.70 to 1.13 mm) using the t test. Regression analysis confirmed the lack of significance (P = .30; −0.36 mm, 95% CI, −1.07 to 0.34).

Figure 3. Mean Warp in the Lateral Plane Over Time for Oblique Split and Concentric Carving Grafts.

Differences between the 2 groups are shown at 4 time points.

At the initial measurement at 1 hour, 1 oblique split graft and 2 concentric carving grafts had noticeably warped to the naked eye. Although in a clinical situation one would not use a warped graft, the decision was made to keep the samples in the experiment because not every surgeon rests grafts for 1 hour before implantation, so it was feasible that these samples would have already been used before the onset of warp.

By 3 months, 2 rib grafts had completely desiccated and broken, and one of them had warped so badly it could not lie in either plane for measurement. These samples were included up until 2 months and then were excluded. In our original study protocol, we had also planned to follow up the grafts at 6 months and 1 year to obtain a picture of longer-term warping, which has not been reported in vitro in the literature, to our knowledge. However, when we reviewed the grafts at 6 months, most of them had desiccated to such a degree as to render them clinically useless, and we decided to terminate the study early.

Discussion

This investigation is the first laboratory study to date to assess the behavior of rib grafts harvested using the oblique split method. Clinical results have previously been reported by a research group in Turkey in 2 articles, where they describe a complete absence of warp in the grafts. Herein, we were not able to replicate this finding in vitro, although it is difficult to know how clinically discernible our amount of cartilage deflection (range, 1.12-1.57 mm) would be in situ within the nose, especially when considering the stabilizing forces of sutures and the soft-tissue envelope. For obvious ethical reasons, few studies have investigated this behavior.

Our results with respect to the concentric carving grafts are in contrast to the work by Farkas et al. Using storage conditions identical to ours, they demonstrated a statistically significant amount of warp from baseline at 1 hour, as well as another increase at 1 month in several types of concentrically carved grafts. The team used computer software to calculate a parabolic coefficient from a quadratic regression derived from a photograph, and it is impossible to know if that value translates into clinically significant warping of the rib or how it compares with our measured data.

We demonstrated that the 30° oblique split method is equivalent to the concentric cartilage harvest technique in terms of the amount of warp experienced. The mean difference in warp between the oblique split and concentric carving groups overall was only 0.18 mm. Although this investigation was an in vitro cadaveric study and the clinical applicability should be interpreted with caution, we believe that should the results be replicated in vivo, a difference in deviation of 0.18 mm would not be materially visible.

While we were unable to confirm a benefit in terms of its warping profile, the oblique split method has several other advantages that we discovered anecdotally. The main advantage is the increased amount of graft material that can be harvested from a given piece of rib. The graft preparation time also appears to be reduced, although this reduction was not quantitatively measured in this study, and the skill required to carve the grafts is arguably less because one need not worry about keeping the correct orientation and ensuring that even amounts are obtained from each side, creating 6 flat surfaces. If the width of the graft is limited by narrow anteroposterior rib dimensions, 2 oblique grafts can be sutured together. Similarly, 2 grafts can be sutured end to end to gain length, or the angle at which the rib is cut could be reduced, although this technique has not been studied experimentally, to our knowledge. This characteristic potentially makes the oblique rib graft more versatile than the concentric pattern of carving.

Limitations

There were some limitations to the study. The age range of our cadavers was not representative of the typical rhinoplasty patient, and the advanced age undoubtedly affected cartilage quality. However, costal cartilage is often required in nasal reconstruction after skin cancer excision, and this population is usually older, with corresponding poorer quality of cartilage. By rejecting any portions of rib that could not be cut with a dermatome blade, we believe that we have mirrored the clinical situation as closely as possible. Ideally, we would repeat the study in a younger cadaver population, although we have been unable to access any such cadavers to date.

Given the size of the grafts, we believed that 10 data points in one plane and 2 data points in another plane would be an accurate representation of the surface contour of the rib. However, it is acknowledged that the true highest and lowest values might not have been captured. It was a necessary balance between having enough data points without adding unnecessary weight to one surface of the rib, which may itself influence warping behavior.

To capture early warp, we would have ideally measured warp immediately and at 1 hour after carving. Unfortunately, this measurement was not possible because of restrictions on the amount of cadaver material and equipment to which we had access. However, as reported, only 3 grafts had noticeably warped during this period. Furthermore, because it is common practice for the operating surgeon to store freshly carved grafts for a short period before implantation and discard or modify any that had visibly warped, it could be argued that precise measurement of very early warp was not pertinent to this study.

We acknowledge that our graft storage arrangement was not representative of in vivo conditions, and we do not know the extent to which this limitation altered the viability of the cartilage and its propensity to warp. In vitro results may not mimic in vivo warping in the long term. The fact that we compared the oblique split method with the current criterion standard concentric carving technique in vitro should mitigate this limitation somewhat and allow better comparison with other in vitro studies, whose study conditions we tried to emulate as closely as possible. We extended our observation for longer periods than previous authors, which also presented challenges. At the 2-month reading, all of the ribs were in good condition. Between 2 and 3 months, a few of the grafts became dry. Extra saline was added to the gauze to moderate this condition, but most had desiccated and were unusable by 6 months.

Conclusions

The best graft material for reconstructive rhinoplasty remains elusive. An ideal graft is easy to harvest while causing minimal morbidity. It is abundant, of good size, and easy to carve. It is reliable, with little resorption, warp, or other changes in property after implantation. Although the harvesting technique and associated risks are the same with the oblique split method and the concentric carving method, the oblique split technique is superior in terms of the quantity of available graft material and ease of carving. Our study demonstrates equivalent warping characteristics between the oblique split and concentric carving methods. Taken together, these findings indicate that the oblique split method represents an exciting new development in reconstructive rhinoplasty and nasal reconstruction. Further studies, ideally using cartilage from a younger population, are required to confirm our results.