Comments on: “Quantitative Evaluation of Mechanical Properties in Tissue-Engineered Auricular Cartilage”

To the Editor:

We read with great interest the recent review by Nimeskern et al. ¹ The study describes the importance and need for quantitative mechanical evaluation of tissue-engineered auricular cartilage (AUC). While this review is comprehensive and important, there are some practical issues that require clarification.

First, the authors initially state that reconstruction with autologous cartilage is the only existing treatment for AUC defects. We do agree that autologous cartilage is the most commonly used implant material, but some clinicians have reported high success rates with synthetic implants, such as porous polyethylene.^2,3 Interestingly, this is noted by the authors later in the review article.

Second, the authors suggest that AUC serves an important functional role, and its elastic nature is crucial to this role; however, to the best of our knowledge, the evidence to support this is lacking. The auricle is involved in filtering sound energy to the external auditory canal and tympanic membrane, along with optimizing the resonance, but this is due to the shape and position of the auricle.^4,5 The intrinsic mechanical properties and flexibility are not known to provide any inherent functional advantages. There are, however, aesthetic concerns associated with AUC defects, which may cause psychosocial problems. ⁶

So, do we truly need a tissue-engineered AUC to mimic the mechanical properties of native ear tissue? Although the goal of generating tissue that resembles native tissue is laudable and consistent with the paradigm of replacing like with like, there is no practical reason when considering AUC. This is clearly not the case when considering other cartilaginous structures in the body. For instance, articular cartilage is involved in joint movements and weight-bearing, and thus, any engineered articular constructs should mimic native mechanical properties. ⁷ The same principle does not apply in AUC tissue engineering due to the lack of function that is related to the intrinsic mechanical properties.

Finally, there are some surgical considerations. Most cases of ear reconstruction involve children with microtia. Typically, the skin must stretch tremendously to accommodate the cartilage construct, which can be quiet bulky. Such high-load stress imparted by the overlying soft tissue onto the framework can cause complications, such as skin erosion and implant extrusion. ⁸ If the implant is not firm enough, the high load and stress may cause partial resorption and change the intricate appearance of the underlying auricular framework. ⁹ This is the main reason why reconstruction is delayed until children are around 10 years of age. By this age, the rib cartilage is firmer and can better withstand the load exerted by the overlying soft tissue.

If we implanted an ear-shaped construct with mechanical properties identical to native AUC, the helical rim, especially at the superior pole, would be hyperflexed toward the mastoid region. This may be avoided by the use of tissue expanders to stretch out the skin before implantation, but tissue expanders are not well tolerated in children.

In summary, researchers should bear in mind the abovementioned points in order for the tissue-engineered AUC to be used effectively in the clinical setting. We remind the readers that a multidisciplinary approach with early involvement of the potential end-users (clinicians) is advisable.