Costal cartilage in a Porcine Model

Analysis of the physical properties of costal cartilage in a porcine model

Manuel A. Lopez, M.D.1, Anil R. Shah, M.D.1, John G. Westine, M.D.1, Kevin O’Grady, B.S.1, Dean M. Toriumi, M.D., F.A.C.S.2

1 Department of Otolaryngology- Head and Neck Surgery, University of Illinois @ Chicago
2 Professor, Division of Facial Plastic and Reconstructive Surgery, Department of Otolaryngology- Head and Neck Surgery, University of Illinois @ Chicago

To whom all correspondence should be addressed, Manuel A. Lopez, M.D. 1855 West Taylor Ave, Chicago, IL 60612, Tele- 312-996-6584, Fax 312-255-8904 or sent by email, Mlopez5@uic.edu

The authors do not have any commercial or proprietary interest in the product discussed in this article.

Abstract
Objective: To determine the impact of interventions on the degree of warping of costal cartilage. 

Methods: The project was conducted at a large university animal research laboratory. The costal cartilage of eight thirty-kilogram domestic pigs was harvested.  The cartilage was cut into central and peripheral segments with a standard cutting dye. Two sizes of rectangular cubes were compared.  The central portions of costal cartilage were segmented and glued with octyl-2-cyanoacrylate.  The shape of the cartilage was documented with both digital and film photography.  The cartilage was placed into subdermal pockets on the dorsum of the pigs.  The animals were sacrificed at 4 weeks and the cartilage was photographed.  Adobe Photoshop software was used to measure the degree of warping. Statistical analysis was calculated by t-test analysis.

Results:  A total of 115 rectangular costal cartilage blocks were treated.  Large blocks warped less than small blocks (p < .024).  Centrally cut blocks warped less than peripherally cut blocks (p< .032).  The octyl-2-cyanoacrylate incited a significant sterile inflammatory response such that the blocks could not be accurately assessed for warpage.

Conclusions:  Costal cartilage can be effectively used for grafting in rhinoplasty with minimal warping if large grafts from the central portion of cartilage are used. 

Secondary rhinoplasty is a difficult operation.  Scar and deformities that the previous surgeon left behind must be dealt with systematically.  Often times it is not what the previous surgeon left, but what was taken away that complicates the rhinoplasty.  In order to correct tip deformities, augment the dorsum and provide structural support to the nose, a large amount of grafting material is often needed.  Deformities such as a severe saddle nose are difficult to correct utilizing only septal cartilage, especially since the cause of the deformity is most often a paucity of septal cartilage.   Autologous costal cartilage provides an abundant amount of grafting material that provides significant resiliency to counteract the inherent forces acting on the nose.  The disadvantage of using costal cartilage is the possibility of warping (tendency to change shape over time) postoperatively.  The tendency to warp does not affect the postoperative course in structural grafts such as spreader grafts or columellar struts; however, costal cartilage that is used to augment the dorsum has little tolerance for warping.  A dorsal graft that has a minimal amount of warping can lead to an obvious external deformity. (Figure 1)

There are a variety of other options for augmenting the nasal dorsum.  Auricular cartilage can be used to augment the dorsum; however, the contour of the cartilage increases the possibility of a visible edge deformity postoperatively.  In addition, auricular cartilage will often times not supply enough material to augment the dorsum and correct tip deformities.  Alloplastic materials have been described for use in nasal reconstruction and there are long-term studies with minimal problems.1  The availability and ease of use are seductive; however, extrusion, infection and the possibility of autoimmune complications can leave the patient with a bigger problem than which they initially presented.2,3  Irradiated costal cartilage has been shown to be a viable option  and provides a significant amount of grafting material without any donor site morbidity;  however, there still remains the question of long-term resorption.4,5,6

The properties of costal cartilage responsible for warping have been examined in previous studies.  Fry demonstrated that protein polysaccharides within cartilage produce internal tensile stresses that cause the cartilage to change shape.7  It is the quest of the rhinoplasty surgeon to control these stresses in order to get a predictable postoperative result.  Gibson and Davis showed that using balanced cross sections could diminish the chance of the cartilage warping.8 By carving the central portion of the cartilage, the stresses are balanced thereby decreasing the chance of warping.  However, even when using the core of the costal cartilage, warping can still occur.  In response to this dilemma, Gunter showed that inserting a metal wire through the cartilage decreased the incidence of warping, but there were problems such as wire extrusion and a tooth root being devitalized.9 

In this study we have attempted to balance the internal stresses by carving the central component of the costal cartilage.  The central symmetrically carved component will be compared to the peripheral components of the costal cartilage.  In addition, we have evaluated whether the cross-sectional area of the graft impacts the tendency of the cartilage to warp.  Lastly, we segmented a subset of graft to assess whether segmenting the grafts, and then re-approximating the segments with cyanoacrylate glue would decrease the amount of warping of the grafts. 

We have hypothesized that by symmetrically carving the central segments of cartilage we will see less warp than in the peripherally cut grafts.  Moreover, we believe that the larger the graft, the more the cartilage is able to withstand the internal forces that cause the warp to occur; therefore the larger segments should warp less than the smaller segments.   Finally, we believe that by interrupting the internal stresses by segmenting and re-approximating the cartilage segments, the degree of warping will decrease.

Methodss
>Eight thirty-kilogram domestic pigs were used in this study.  The operations were done under general endotracheal anesthesia under the guidance of a licensed large animal veterinarian.  The procedure was performed under sterile conditions.   The chest and abdomen of the pigs were prepped with povidone-iodine solution.  Bilateral 10 cm subcostal incisions were made and the cartilaginous component of the inferior confluent ribs of the domestic pigs was harvested.  The ribs were then completely stripped of perichondrium.  The cartilages were carved into two sizes of rectangular cubes, a 4 x 4x 20 mm block and a 2 x 2 x 20 mm block. This was done using a cartilage–cutting device similar to that described in previous studies. (Figure 2) 9,10 

The 2 x 2 x 20 mm cartilage blocks were divided into the following groups:

1. Centrally cut
2. Peripherally cut
3. Centrally cut, segmented, and glued
4. Peripherally cut, segmented, and glued

The 4 x 4 x 20 mm cartilage blocks were divided into the following groups:

1. Centrally cut
2. Centrally cut, segmented, and glued

The blocks of cartilage that were segmented and glued were cut perpendicular to the longitudinal plane with a No. 15 Bard Parker blade at 5mm increments.  The octyl-2-cyanoacrylate glue was placed between each segment.  The segments were held in place until the glue was fixed.

 Digital as well as 35mm photographs were taken at a fixed distance from the specimens.  The cutting dye carved a rectangular cube of cartilage such that the side of the cartilage that was photographed was irrelevant.  

The pigs were then placed in the prone position and the dorsum was prepped with povidone-iodine solution.  The specimens were inserted into subdermal pockets on the dorsum of the domestic pigs. The pockets were labeled using a tattoo gun.  The domestic pigs were then housed according to the large animal care committee of the University of Illinois @ Chicago protocol.  Four weeks from the intervention, the pigs were sacrificed and the cartilage blocks were harvested. 

Digital and 35 mm photographs were taken at the same distance from the specimens as was originally taken.   The angle of warpage was calculated using Adobe Photoshop software by using the measure tool.   The angle of warpage is defined by the angle created by two lateral portions of the rib connected with the point of maximal warpage. 

Statistical analysis was calculated by a paired t-test for means for pre and post treatment differences within treatment groups and comparison between treatment groups was calculated by comparing a one-tailed t-test, two-sample of unequal variances. A p value less than .05 was considered significant.

Histological evaluation was done to assess the impact of the cyanoacrylate glue on the surrounding tissue versus the non-segmented specimens. The samples were fixed in paraffin and stained with hematoxylin and eosin. 

Results
A total of 115 rectangular costal cartilage blocks were treated according to group assignment resulting in 26 large central, 26 cut central, 17 small central, 15 small central cut, 15 small peripheral, 17 cut small peripheral blocks respectively.
(Table 1)

A comparison was made at time 0 and time 4 weeks within each treatment group to determine if there was a statistically significant degree of warping.  In the large central core group, the time 0 angle of warpage mean (176.8) did not differ significantly from the time 4 weeks warpage mean (177.1).  However, in the small central core group there was noted to be a difference between time 0 (176.8) and time 4 weeks (174.4, p<.018).  There was also noted to be a difference in the small peripheral group warping mean at time 0 (176.7) and at time 4 weeks (171.0, p<.002).

In order to analyze the impact of the various treatment modalities on warping, the warping differences between times 0 and time 4 weeks were calculated within groups and compared.  The warping differences are calculated values which demonstrated an increase in warping between time 0 and time 4 weeks.  In the groups were warping was not observed, the value of 0 was assigned.  Comparing the mean difference between small peripheral (6.47) and small central (3.00) treatment groups with a one-tailed t-test revealed a significant difference between the two treatment modalities (p<.032).  Similarly, a comparison between the small central treatment group (3.00) with the large central group (1.12) also revealed a statistically significant result (p<.024).

Gross evaluation of the treatment groups receiving cyanoacrylate glue revealed resorption of cartilage and fibrosis of the surrounding tissue that displaced the segmented cartilages.  Histological evaluation confirmed these findings with microscopic erosion and a proliferation of inflammatory cells surrounding the cyanoacrylate glue. (Figure 3)

Due to the reactive changes noted, statistical measures and analysis were not obtained.

Discussion
Von Mangoldt first described in 1889 utilizing autologous costal cartilage as grafting material for nasal reconstruction.11  Long-term follow-up has shown that costal cartilage in nasal reconstruction provides durable bulk with minimal resorption.12  Autologous costal cartilage provides a large amount of grafting material that is often needed in secondary rhinoplasty.   Costal cartilage is easily contoured and rigid enough to provide structural support to the tip, yet the possibility of warping  can complicate the postoperative course. 

A few causes of a low nasal dorsum include traumatic disruption of the septal cartilage with or without a septal hematoma/abscess that results in a classic saddle nose deformity, over-aggressive reduction of a cartilaginous or bony hump, or a congenitally low dorsum.  The therapeutic options to correct this deformity are numerous.  Autologous costal cartilage is the best tolerated grafting material with the least incidence of infection and extrusion.  The resorption rates of cartilage have been previously studied and have been shown to be low.13 However, costal cartilage does have the propensity to warp.  When the graft is placed on the nasal dorsum, warping can lead to a significant physical deformity.  Various techniques have been described to attempt to reduce the incidence of warping.14 The senior author has routinely utilized the central portion of costal cartilage when using it to augment the nasal dorsum.

This study is the first to quantitatively evaluate the degree of warping of costal cartilage in an in vivo model.  Earlier studies have evaluated costal cartilage in an in vitro model to assess the factors that affect warping and the rate at which the warping occurs.9,10  Gibson and Davis reviewed 46 patients that had undergone costal cartilage augmentation of the nasal dorsum and determined that no warping had occurred.8   These authors carved the costal cartilage grafts such that the graft inserted was a central portion of cartilage.  The method of evaluation was a review of the patients’ charts indicating if warping had occurred clinically.  The benefit of this study is that we were able to directly measure the degree of warping in costal cartilage and determine if there was a difference in the amount of warping dependent on the intervention to the costal cartilage. 

We found that there was a significant difference in the degree of warping when cartilage was cut from the periphery vs. the central portion.  This agrees with the hypothesis that the internal subperichondrial forces of costal cartilage must be balanced in order to eliminate warping.  Clinically it is recommended to carve the costal cartilage in a balanced manner and allow the cartilage to soak in a saline basin.  Periodically evaluate the cartilage for areas of early warping and compensate for the warping by carving the cartilage on the concave side.  This process should be continued until a straight piece of cartilage is obtained, and then the graft can be placed with a high degree of certainty that it will unlikely warp.

We also found that the smaller the cross-sectional area of the costal cartilage graft was the more likely it was to warp.  This is important because most patients that benefit from costal cartilage augmentation of the nasal dorsum require significant augmentation.  In situations that the patient only needs slight augmentation, auricular or septal cartilage may be the preferred grafting material due to the higher probability of costal cartilage to warp.

The third portion of our study attempted to disrupt the internal subperichondrial forces of the costal cartilage.  The costal cartilage grafts were cut into segments and glued together using octyl-2-cyanoacrylate.  When compared to the non-segmented grafts, the segmented and glued grafts displayed a significant amount of soft tissue inflammation.   The histotoxicity of cyanoacrylates has been shown in previous studies.15,16   We attempted to control the amount of cyanoacrylate that contacted the surrounding soft tissue by using minimal amounts.  However, the slightest amount of cyanoacrylate initiated a significant sterile inflammatory response.  The intensity of the response was so great that the grafts that were segmented and glued were unable to be accurately measured for warping.  In response to the significant histotoxicity, we feel that cyanoacrylates should not be utilized to fix cartilage within the soft tissue of the nose.

A shortcoming of this study is that although this is an in vivo model, the inspiratory and expiratory forces that act on the nose were not duplicated.  The grafts were purposefully placed on the dorsum of the pigs so that external forces acting on the cartilage would be minimized.  Therefore, the degree of warping would be primarily due to the intrinsic forces of the costal cartilage.   A human study would need to be done in order to incorporate the inspiratory and expiratory forces.  Unfortunately, the possibility of that type of study is unlikely given the cosmetic deformities that may result.  A better understanding of the physical properties of costal cartilage is still needed, but based on this study and preceding studies, the warping phenomenon can be minimized by carving symmetrically from the center of the rib and utilization of large cross-sectional areas.

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Table 1

NC- not calculated secondary to significant inflammatory response