Atrophy of Masticatory Muscles in TMD Patients

Abstract

Temporomandibular disorders (TMD) are characterized by a substantial amount of pain in the temporomandibular joint and all four surrounding muscles of mastication. While myofascial pain associated with TMD has a relatively high prevalence (6% in the US), researchers have still been unable to indicate the cause and progression of TMD. Caspase-dependent apoptosis is commonly associated with many human muscle pathologies. Although apoptotic mechanisms have been commonly applied to accepted mechanisms of skeletal axial muscle atrophy in both human and murine samples, the same has not generally been applied to proposed models of masticatory muscle atrophy. While murine samples were found to exhibit endoplasmic reticulum (ER) stress which caused an accumulation of misfolded proteins thereby activating cleavage of ER localized caspase-12, human samples may show an analogous model with caspase-4.

            The muscles of mastication, as a specialized group of craniofacial muscles that retain embryonic fiber properties in adults, suggest that developmental origin of these muscles may govern a pattern of expression that differs from limb muscle. This study examines human masticatory muscle samples through an evaluation of histology and molecular analysis.  Muscle samples were collected from healthy individuals (control) and individuals with diagnosed temporomandibular joint disorder in order to match diagnosed TMD with histological muscle degeneration.

Dr. Giannakopoulos diagnosed all of the patients as either TMD patients or non-TMD patients using guidelines outlined in the 2008 New England Journal of Medicine review article, "Temporomandibular Disorders." Study inclusion criteria includes: Age > 18, clinical/radiographic evidence of TMD, diagnosis of TMD (myofascial pain and/or internal derangement), undergoing TMJ surgery and with available masticatory muscle specimen (masseter, temporalis, lateral pterygoid or medial pterygoid) which would otherwise be discarded, and no compromising systemic conditions.

Twelve total samples were collected. Two samples from each patient were removed from the same masticatory muscle, one to be fixed in formalin for histology, the other preserved in RNA Later for future molecular analysis. Histology samples were cut into cross sections and immunofluorescentally stained with laminin.

            Staining results revealed all control samples have no histological signs of TMD while all TMD samples show signs of stalled regeneration – see figures 1 and 2, respectively.

          

In conclusion, the samples Dr. Giannakopoulos diagnosed clinically as non-TMD and TMD are consistent with the histological findings of non-TMD samples that lack stalled regeneration versus TMD samples that contain stalled regeneration, respectively. This study has potential to evaluate the levels of genes associated with apoptosis in non-TMD vs TMD samples. Based on previous mouse studies (Evans et al, 2008), heightened expression of stress genes was found in the murine masseters, and ER stress was evident. The authors may extend this research to human samples through RNA profiling to determine if there are heightened stress gene expression in muscles from TMD patients, or if there are no differences between non-TMD versus TMD samples.  As the diagnosis of TMD is consistent with the myofascial histology obtained, the authors are presented with the opportunity to use the corresponding samples that have preserved RNA in order to search for markers of myofascial degeneration or stress present in TMD.  One promising way would be to harness the power of molecular profiling to screen for thousands of genes.  Doing so could help develop drug targets and potential therapies in the already complex treatment of TMD. 

Works Cited:

1. Barton ER. Expression profiling

reveals heightened apoptosis and supports fiber size economy in the

murine muscles of mastication. 2008

2. Moorwood, Caspase-12 ablation preserves muscle function in the mdx mouse. 2014.