Table 2.

Experimental approaches in PDL research.

Reference	Year	Species	Method	Factors impacting PDL response
Reitan [33, 72]	1957	Human	in vitro	Applied force magnitude and type (continuous versus intermittent), mechanics involved (tipping versus bodily movement), and individual patient variation in tissue reaction
	1964
Reitan [78]	1967	Human	in vitro	Density, supraalveolar fibers, structure of collagen fibers and cellular activity in the PDL. Force/unit of root surface area (on response rate)
Mitchell et al. [79]	1973	Cat	in vitro	Individual tooth types
Chiba et al. [80]	1981	Rat	in vitro	Adrenocorticoids (drug)
Ohshima [81]	1982	Rat	in vitro	Lathyrogens (drug)
Komatsu et al. [82]	1988	Rat	in vitro	Occlusal conditions
Ashizawa and Sahara [83]	1998	Rat	in vitro	Stress found to vary significantly in different segments and PDL thickness also changed with the remodeling of the alveolar bone during treatment
Toms et al. [84]	2002	Human	in vitro	Age, disease state (health), anatomical location of tooth root, teeth (premolar, canine, incisor), arch (maxillary, mandibular) and fiber orientations
Dorow et al. [85]	2003	Pig	in vitro	Young's modulus depended on loading velocity. This meant stiffness of the PDL increased with loading velocity—conforming to studies [86–88]
Kawarizadeh et al. [89]	2003	Rat	in vitro	Fresh versus frozen specimens
Komatsu et al. [28]	2004	Rat	in vitro	Advancing age enhanced PDL's mechanical strength and toughness (mostly incisal region) and decreased viscous fraction (incisal and basal regions) along the incisor's long axis
Komatsu et al. [90]	2004	Rat	in vitro	Maximum shear stress and stiffness decreased with age; toughness unchanged (>extensibility)
Sanctuary et al. [25]	2005	Cow	in vitro	Species, location, strain history, and strain rate. Strain rate was also suggested by Natali et al [91]
Tanaka et al. [92]	2007	Pig	in vitro	Preparation of specimens and location in mouth. Nonlinearities, compression/shear coupling, and intrinsic viscoelasticity affected shear material behaviour (important implications for load transmission from tooth to bone and vice versa)
Genna et al. [93]	2008	Pig	in vitro	PDL's small size and complex microstructure; PDL sample preparation, sample cutting, with associated damage to inclined fibres; sample freezing; presence/absence of pressurized fluids during tests; difference in results taken from different teeth or root positions along the same tooth; sample orientation and fibre inclination
Qian et al. [94]	2009	Pig	in vitro	Deformation patterns in entire periodontium depended on geometrical profiles and material properties—especially PDL
Pilon et al. [95]	1996	Dog	in vivo	Differences in bone density, bone metabolism, and turnover in the PDL. Force magnitude was NOT decisive in determining the rate of bodily tooth movement
Komatsu et al.  [96]	1998	Hamster Mouse Rabbit Rat	in vivo	Species, strength, and stiffness of the periodontal collagen fibers and PDL waviness and thickness depended on developmental stages of the periodontal collagen fibers possibly related to the general arrangement, diameters and collagen fiber bundle densities, and fiber insertions into the alveolar bone and cementum. Dynamic shear moduli increased nonlinearly with frequency—regardless of the magnitude of applied strain (implies that PDL stiffness increases with frequency)
Tanne et al. [41]	1998	Human	in vivo	Adult Young's modulus (PDL) was greater than that of adolescents. [97, 98] showed similar results. This might lead to delay in adult tooth movement from a reduction in the PDL's biological response
Jones et al. [64]	2001	Human	in vivo	Age and periodontal health
Yoshida et al. [66]	2001	Human	in vivo	Load magnitude