Biomechanical comparative analysis of temporomandibular joint, glenoid fossa and head of the condyle of conventional models prothesis with new PEEK design

Abstract

This article reviews the design of the temporomandibular joint (TMJ) prostheses used for TMJ joint replacement from 2000 to 2020. The TMJ is a complex joint, with distinct anatomical and functional characteristics making it challenging to maintain. Many authors from the early 20th century reported techniques for TMJ reconstruction, with the goal of restoring its shape and ideal function. Many prototypes have been developed in pursuit of an ideal prosthesis that adheres to the principles of biomechanics and biocompatibility, with good long-term performance and lower cost.

The TMJ prosthesis is divided in two parts: the glenoid fossa and the mandibular ramus component.

These two parts are fixed with metal screws in the glenoid fossa and fixed in the zygomatic arch with 4 or 5 screws. The mandibular part is fixed to the mandible ramus with 8 or 9 titanium screws.

In our review, since 2000 to 2020, little has changed to improve the design and allow for natural mandible movement. From 2000 to 2006, 48 TMJ surgeries were performed using UHMWPE with this design. All patients had good results, preserving opening mouth and lateral movements.

All the designs are similar in principle. The glenoid fossa, which resembles a box, limiting the rotation and translation movement. It is known that lateral movements are lost in function as the lateral pterygoid muscle is detached.

1. Material and methods

This study reviewed 30 years of the literature and clinical experience with TMJ prostheses. We conclude that many materials have been used to manufacture TMJ prostheses, and today all prostheses are made with the same material as in orthopedic prostheses, UHMWPE for the glenoid fossa and titanium, cobalt chrome and molybdenum for the mandibular body and head. All glenoid fossa have the same design.

The movements of the temporomandibular joint were calculated using measurements and geometric functions, typed and represented in a computer spreadsheet using the program Image, Processing and Analysis in Java, version 1.8.0, compatible with Windows 10. Reviewing the glenoid fossa design, serious problems were observed due to its box-like design. This structure limits the rotation-translation movement, frequently causing trauma with the head of the mandible against the anterior wall of the glenoid fossa prosthesis, working like a stop. When the patient opens his/her mouth beyond the limit, dislocation occurs, causing a serious problem for the patient. This occurs because the forces of the temporal and masseter muscles involved in the action of closing the mouth and the condyle will lock at the anterior wall of the glenoid prosthesis, making it impossible to close the mouth (dislocation occurs).

The forces of the muscles, such as the masseter, temporal, medial pterygoid and supra- and inferior hyoid muscles, during open mouth movement will be abruptly stopped at the anterior wall of the glenoid fossa. The force of these muscles will be transferred to the mandibular ramus prostheses, causing excessive pressure during its movement. This force will be directly transferred to the screw area of the ramus of mandibular prosthesis, allowing micromovements around the fixation screws. This will cause bone reabsorption around the screws, loosening the fixation.

Mercuri et al. reported on missing screws in the stock TMJ ramus prostheses in the process of bone resorption.¹ Van Loon et al.² reviewed TMJ prostheses and concluded that the design and material do not provide good results for patients, and the period of usage is short (8–12 years). In 2000, we began designing a new TMJ prosthesis, using Au (89%) with Ag (11%) for the mandibular ramus, and UHMWPE for the Glenoid fossa. A pilot study was conducted, and after 5 years follow-up, no complications or subluxation occurred (Table I). In 2008, new material, PEEK Lt1 20% Ba, was used to develop a TMJ prosthesis system with the same design. (Table I).

Table 1.

Date of references from patients who underwent Total TMJ Reconstruction (Prothesis) with gold (Au) and silver (Ag) materials and ultra-high-weight polyethylene (UHMWPE) (n = 10). Total Surgeries (n = 32). Source: Chart from Hospital 9 de Julho (São Paulo – Brazil) and Hospital IGESP (São Paulo – Brazil) from 2000 to 2010.

NAME	AGE	GENDER	YEAR SURGERY	LATERAL	LATERAL/MOV INITIAL(R/L)	LATERAL/MOV FINAL(R/L)	PROSTEHESES MATERIALS	OPEN INITIAL	OPEN FINAL	FOLLOW-UP 3 MONTHS	FOLLOW-UP 6 MONTHS	FOLLOW-UP YEARS	COMPLICATIONS
1.E.	49	F	2002	Unilateral	0.3 mm	0.6 mm	Au(89%)+Ag(11%)+UHMWPE	15 mm	35 mm	X	X	5	–
2.M.	53	M	2001	Unilateral	0.2 mm	0.4 mm	Au(89%)+Ag(11%)+UHMWPE	16 mm	35 mm	X	X	5	–
3.M.	19	F	2002	Bilateral	0.1 mm	0.6 mm	Au(89%)+Ag(11%)+UHMWPE	10 mm	45 mm	X	X	3	–
4.R.	55	F	2002	Bilateral	0.0 mm	0.4 mm	Au(89%)+Ag(11%)+UHMWPE	0.5 mm	32 mm	X	X	4	Fracture of Mandibular component
5.K.	30	F	2000	Unilateral	0.0 mm	0.4 mm	Au(89%)+Ag(11%)+UHMWPE	20 mm	38 mm	X	X	5	–
6.E.	60	F	2004	Unilateral	0.3 mm	0.5 mm	Au(89%)+Ag(11%)+UHMWPE	15 mm	35 mm	X	X	6	–
7.Z.	58	F	2001	Bilateral	0.0 mm	0.4 mm	Au(89%)+Ag(11%)+UHMWPE	20 mm	37 mm	X	X	9	–
8.V.	49	F	2000	Bilateral	0.0 mm	0.4 mm	Au(89%)+Ag(11%)+UHMWPE	0.5 mm	30 mm	X	X	6	–
9.J.	50	M	2004	Unilateral	0.3 mm	0.5 mm	Au(89%)+Ag(11%)+UHMWPE	20 mm	35 mm	X	X	5	–
10.J.	20	M	2001	Bilateral	0.0 mm	0.5 mm	Au(89%)+Ag(11%)+UHMWPE	0.2 mm	30 mm	X	X	5	–
11.M.C.	59	F	2013	Unilateral	0.4 mm	0.6 mm	PEEK	18 mm	30 mm	X	X	5	–
12.M.S.	61	F	2013	Bilateral	0.3 mm	0.6 mm	PEEK	20 mm	40 mm	X	X	10	–
13.S.	53	F	2013	Bilateral	0.0 mm	0.5 mm	PEEK	20 mm	30 mm	X	X	10	–
14.I.	40	F	2014	Bilateral	0.0 mm	0.5 mm	PEEK	15 mm	30 mm	X	X	8	–
15.A.	42	F	2014	Bilateral	0.0 mm	0.6 mm	PEEK	10 mm	35 mm	X	X	8	–
16.J.	25	F	2014	Bilateral	0.0 mm	0.6 mm	PEEK	20 mm	40 mm	X	X	8	–
17.K. S.	35	F	2016	Bilateral	0.0 mm	0.5 mm	PEEK	18 mm	40 mm	X	X	6	–
18.I.	28	F	2015	Bilateral	0.2 mm	0.6 mm	PEEK	22 mm	40 mm	X	X	7	–
19.M. Co.	70	F	2013	Bilateral	0.2 mm	0.4 mm	PEEK	25 mm	43 mm	X	X	5	–

PEEK is a polymer (polyether ether ketone) derived from petroleum and developed by Invibio (Lancashire, UK). Marketed in the 1980s. It is a thermoplastic polymer, consisting of an aromatic molecular structure, interconnected to a ketone and linked to other functional groups. It was previously used for orthopedic and spinal implants. Its molecular structure provides resistance to high temperatures, providing stability, being an inert compound. It is compatible with reinforcing agents such as glass and carbon fibers with greater resistance than many metals. In 1990, many researchers evaluated the biocompatibility and in vivo stability of PEEK, classifying it as a high-performance material. With a modified design of the glenoid prosthesis and the ramus, a new TMJ prosthesis was developed. The glenoid fossa is flattened.

As shown in Table II, the mean age of the patients (N = 19) was 45.5 years old, the majority were females (84.21%). From 2000 to 2007 we did more surgeries, most cases involved a bilateral approach (68.42%); the total of surgeries was 32, between uni and bilateral in 19 patients. The opening in the mouth before the surgery was 15.57 mm and after the surgery was 35.78 mm. With the modified surgery technique, all patients had preserved lateral movements (right/left), from 1.21 mm before and 5.05 mm after the surgery.

Table 2.

Analysis and descriptive statistic from the study variables in perceptual and average datas from patients who underwent total TMJ prosthesis surgery.

VARIABLE	%	n	Average
Age	–	19	45,5
Male	15,78	03	–
Female	84,21	16	–
Surgery 2000–2006	52,63	10	–
Surgery 2007–2013	21,05	04	–
Surgery 2014–2020	26,31	05	–
Unilateral	31,57	06	–
Bilateral	68,42	13	–
Lateral movements (R/L) Milimeters – Initial	–	–	1,21
Lateral movements (R/L) Milimeters Final			5,05
PT – TMJ Au/(89%) + Ag (11%) + UHMWPE	52,63	10	–
PT – TMJ PEEK – LTI 20% Ba	47,36	09	–
Initial Open Milimeter	–	19	15,57
Final Open Milimeter	–	19	35,78
Follow up - Years	–	19	6,61
Complications Yes	05,26	01	–
Complications No	9	18	–

Due to the high cost of gold, the project had to be stopped; all these patients continue to be monitored until today. With the gold prosthesis we had, (52.63%). Subsequently, we found PEEK LtI 20% Ba, from Invibio, UK (Fig. 1).

Fig. 1.

TMJ gold prosthesis and UHMWPE glenoid fossa.

In 2008, we started studying Invibio's materials, and in 2010, Laros LTDA., developed a TMJ prosthesis in PEEK LtI 20% Ba, after all laboratory tests were completed, we decided to start a clinical study (2013). The PEEK LtI 20% Ba has all the properties that we needed for it, with PEEK LtI 20% Ba, we had (47.38%) surgeries. Our follow-up until this moment presented good results; two cases had bad results (Gold and PEEK LtI 20% Ba) (broken prosthesis ramus body, 6.66%). From 2000 to 2022, with the gold system, and from 2013 to 2022 with the PEEK LtI 20% Ba system, (94.73%) weobtained good results.

According to Wolford et al.,³ the materials should comprise: “1) biocompatible materials; 2) functional compatibility of materials; 3) low wear, flow, and fatigue coefficients; 4) adaptability to anatomic structures; 5) rigidly stabilized components; 6) corrosion resistance and non-toxicity; 7) malleability to facilitate adaptability to the anatomic structures; 8) a posterior stop in the fossa component; and 9) close tolerance of the screw and prosthesis hole diameter.” We would add to this list that “it must have the same subchondral bone compressibility” (Genovesi,W).

PEEK has all these properties; it is an optimal material for orthopedics and maxillofacial devices. It is used often in orthopedic surgery and neurosurgery with interference screws, cages, and anchors for sutures. It is 100% biocompatible and composed of the same subchondral bone compressibility; it is nontoxic and has all the requisite properties.

A prototype TMJ prosthesis was developed with this material. After exhaustive testing done in a laboratory, it performed exceptionally well and had no damage on its surfaces after multiple laboratory tests.

Seventeen pilot surgeries were done in 9 patients. The first surgery was done in 2013 and the last one in 2016. In our retrospective study with the PEEK LtI 20% Ba prosthesis using a new model of glenoid fossa design and mandibular component, the evaluation of patients was satisfactory. The surgery technique was modified, with less trauma for the patient.

All movements were preserved in all patients, including opening, closing, protruding, and lateral movements. The following factors were critical to achieving this outcome:

• 1The weight of the material
• 2The surgery technique
• 3Physiotherapy before and after surgery

Understanding the muscle forces and knowing that the superficial fiber action from temporal, masseter and medial pterygoid muscles support the lateral movements, considering the weight (3.5Gr), of the TMJ PEEK LtI20%Ba prostheses system, all three muscles are able to help carry out the lateral movements.

Studying muscle actions – dynamics of the masticatory system.

According to Koolstra et al.,¹ as shown below, we can represent in a schematic overview the possible actions generated by force Fm of the superficial masseter or anterior temporalis muscle viewed in the horizontal plane (A and B), as forces acting on the lower jaw in the sagittal plane (C). We can also consider the intra-articular distance within the TMJ during free and loaded movements according to Van Loon et al. (1994).

Fig. 1 (A) Possible rotations: Rg, about center of gravity; Rr, about a vertical axis behind the right joint for a laterodeviation to the right; and Rl, about a vertical axis behind the left joint for a laterodeviation to the left. (B) Influence of joint forces: am, moment arm of muscle force; Frj, right joint force; arj, moment arm of right joint force; Fjl, left joint force; and ajl, moment arm of left joint force. (C) Crosshairs: center of gravity. Fclosers: mean force of the jaw-closing muscles. Fopeners: mean force of jaw-opening muscles. Fjoint: joint force. Fbite: bite force. a: moment arm of the different forces.¹ (see Fig. 1a)

Fig. 1a.

(A) Possible rotations: Rg, about center of gravity; Rr, about a vertical axis behind the right joint for a laterodeviation to the right; and Rl, about a vertical axis behind the left joint for a laterodeviation to the left. (B) Influence of joint forces: am, moment arm of muscle force; Frj, right joint force; arj, moment arm of right joint force; Fjl, left joint force; and ajl, moment arm of left joint force. (C) Crosshairs: center of gravity. Fclosers: mean force of the jaw-closing muscles. Fopeners: mean force of jaw-opening muscles. Fjoint: joint force. Fbite: bite force. a: moment arm of the different forces (Koolstra, 2002).

We can also observe in Fig. 2, typical normalized data leading to slopes, showing regressions from five biting moments for a given participant's muscle during bite-force measurement and modeling as reported in Nicket et al. (2012).

Fig-2.

The horizontal axis provides normalized bite forces (N), and the vertical axis provides the corresponding normalized root-mean-square (RMS-mV) muscle activities. (E) Force vectors involved in numerical models of static biting in humans: Applied bite force (100 units), joints (Fcondyle), and representing five muscle pairs (M1,2 = masseter, M3,4 = anterior temporalis, M5,6 = lateral pterygoid, M7,8 = medial pterygoid, M9,10 = anterior digastric muscles), and the axis system used to characterize relative positions of the condyles, teeth, and muscle vectors, based on an individual’s anatomy, are shown (left). Enlargement (right) shows how bite forces were modeled to mimic in vivo biting tasks and characterized by the azimuth angle (θXZ, 0-359°), measured parallel to the occlusal plane, and the angle relative to vertical (θY, where 0° is normal to the occlusal plane) (modified from²).

The horizontal axis provides normalized bite forces (N), and the vertical axis provides the corresponding normalized root-mean-square (RMS-mV) muscle activities. (E) Force vectors involved in numerical models of static biting in humans: Applied bite force (100 units), joints (Fcondyle), and representing five muscle pairs (M1,2 = masseter, M3,4 = anterior temporalis, M5,6 = lateral pterygoid, M7,8 = medial pterygoid, M9,10 = anterior digastric muscles), and the axis system used to characterize relative positions of the condyles, teeth, and muscle vectors, based on an individual's anatomy, are shown (left). Enlargement (right) shows how bite forces were modeled to mimic in vivo biting tasks and characterized by the azimuth angle (θXZ, 0–359°), measured parallel to the occlusal plane, and the angle relative to vertical (θY, where 0° is normal to the occlusal plane) (modified from ²

Our analysis study about the design of the glenoid fossa is the same analysis for the patients having subluxation. Eminectomy surgery is done to improve the patient function and free mandible movement. The studies ^{5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21}, below shows this (see Fig. 2a):

Fig. 2a.

(A) Scopia showing the prosthesis dislocation and (B) Trans operation dislocation; see the white arrow.

Biomechanical analysis of decomposition of tensional forces related to conventional total TMJ glenoid fossa prothesis and in a PEEK mode.

For the biomechanical study of the TMJ involved in the manufacture of total joint prostheses, the normal components involved in the maxillary TMJ movement of the TMJ will initially be used as a reference, since it arises from the intersect between the point of greatest pressure between the head anterior of the condyle of the mandible and the articular eminence. For this purpose, in the configuration reference, the origins and insertions of the muscle slips and the anterior inclination of the articular eminence fixed at 40° in relation to the occlusal plane will be adopted, as well as the joint reaction forces that were estimated with a contact algorithm, according to Fig. 3.^1,3,4

Fig. 3.

Model adapted and modified from the graphical representation of the biomechanical model, as published by Tuijt et al. (2012). Right profile view. The arrows indicate the forces (right side only). F: cesspool. C: mandibular condyle. E: joint eminence. JRF: joint reaction force. Curved arrows: rotational working line of opening/closing the jaw on the axis of the sagittal plane (15° clockwise rotation and 15° counterclockwise rotation), as an example for all muscles. ASA: angle of anterior inclination of the joint eminence. dAP: horizontal distance in the anterior/posterior direction between the center of the condyle and the apex of the joint eminence.

Model adapted and modified from the graphical representation of the biomechanical model, as published by Tuijt et al. (2012). Right profile view. The arrows indicate the forces (right side only). F: cesspool. C: mandibular condyle. E: joint eminence. JRF: joint reaction force. Curved arrows: rotational working line of opening/closing the jaw on the axis of the sagittal plane (15° clockwise rotation and 15° counterclockwise rotation), as an example for all muscles. ASA: angle of anterior inclination of the joint eminence. dAP: horizontal distance in the anterior/posterior direction between the center of the condyle and the apex of the joint eminence.

2. Analysis of vector kinetics

As shown in Fig. 4, we observe the normal situation of the wide open jaw (A) where the moment of joint reaction forces (JRF) exceeds the moment of opening of the jaw of the resulting muscular forces, the result of the vector force that blocks the TMJ caused by the mechanical resistance of the joint eminence. In this case, in condition A and B after the subtraction of the vector forces (red arrows), which determined the resulting vector (blue arrow), we observed a vector force (green arrow) in favor of closing and maintaining the TMJ opening, respectively; that is, generating joint reaction forces (FRA) caused by the resistance of the TMJ eminence at both times: both in the anterior and posterior edges in the mandibular branch, during the excursion of the TMJ movements and demonstrated in condition C in Fig. 4.

Fig-4.

Model adapted and modified from the graphical representation of the biomechanical model, as published by Tuijt et al. (2012). Biomechanical analysis of the vector kinetics of mandibular movements. 1) in attempts to open and close the mandible as a function of the anterior inclination angle (ASA), lower ASAs in darker shades of gray. A: the horizontal distance in the anterior/posterior direction (dAP in mm) between the center of the condyle and the apex of the joint eminence with time. B: the opening angle of the mandible in relation to the upper mandible (degrees) with time. C: joint forces resulting from mandibular movements.

Model adapted and modified from the graphical representation of the biomechanical model, as published by Tuijt et al. (2012). Biomechanical analysis of the vector kinetics of mandibular movements. 1) in attempting to open and close the mandible as a function of the anterior inclination angle (ASA), lower ASAs in darker shades of gray. A: the horizontal distance in the anterior/posterior direction (dAP in mm) between the center of the condyle and the apex of the joint eminence with time. B: the opening angle of the mandible in relation to the upper mandible (degrees) with time. C: joint forces resulting from mandibular movements.

3. Distribution of pressure points in conventional prosthetic components

Research on TMJ prosthesis has been carried out for over 50 years. Initially, there was only a replacement of the semijoint for the condyle or joint fossa. However, this approach was gradually eliminated due to fossa perforation or condylar absorption caused by friction between the bone and the prosthesis.⁵

As shown in Fig. 6, the stress distributions in the prosthesis components were investigated in detail, shown according to the theory and reference module “Von Mises” in the pit components by the Megapascal units (Mpa), which is the standard pressure and tension unit, which is equivalent to the force of 1 N applied uniformly over a surface of 1 m², which are represented by the International System of Units (SI).

Fig-6.

Model from the graphical representation in the figure (A), as published by Oruba et al. (2019) and model adapted and modified with graphic representation using the figures as source (B) and (C), as published by Elledge et al. (2019). Schematic representation (A) of the method used to measure the angle of inclination of the joint eminence (IEA) according to Csado et al. (2011). B and C: IEA angle represented in TMJ - Concepts prostheses with 41.33 degrees and Prostheses in PEEK with 16.98 degrees, respectively. The program used was ImageJ (Image Processing and Analysis in Java – version 1.8.0).

The maximum surface tension of TMJ - Concept components, according to Chen et al.⁶) reached a stress of 19.61/Mpa for the articular fossa (B, above), 170.01/Mpa for the anterior and posterior branches, respectively. The Posterior part of the mandible (B, below) and from 236.08 to 157.39 MPa, for the middle one-third of the vertical axis of the 3 fixing screws (C) located in the upper portion of the prosthetic structure of the mandible. This indicates that masticatory forces can impair stability and rotation movement within the TMJ joint fossa.

The analysis of the decomposition of the joint reaction forces (TMJ) according to Fig. 6 (A), demonstrates that the zone of greatest stress/tension of the TMJ-Concepts mandibular prosthetic component, coincides with that resulting from the 2 vector forces (FRA) as a kinetic response to the eminence of joint strength, in response to the opening and closing movements of the TMJ analyzed biomechanically. The tension responses of the other components in Mpa at the points of greatest stress tb correspond to the regions of greatest tension in the articular fossa (B) and in the fixation screws (C) close to the region of greatest tension in the prosthetic mandibular component (B), mainly at the moment in which the mandible keeps the prosthetic condyle in contact with the anterior region of the fossa and joint eminence, during the opening movements of the TMJ above the 40-degree ASA (Fig. 5).

Fig-5.

Model adapted and modified from the graphical representation of the biomechanical model (A), as published by Tuijt et al. (2016) and Tension stress forces (B and C), as published by Chen et al (2018). A: Vector forces of the joint reaction of the eminence of the TMM resulting from joint biomechanics (model adapted and modified from Chen, 2018). B: Tension stress forces of “Von Mises” represented on the external surface of the prosthetic components above the articular cavity (fossa), mandibular (below) and C: TMJ fixing screws – Concepts.⁶

Model adapted and modified from the graphical representation of the biomechanical model (A), as published by Tuijt et al. (2016) and Tension stress forces (B and C), as published by Chen et al.⁶ A: Vector forces of the joint reaction of the eminence of the TMM resulting from joint biomechanics (model adapted and modified from .⁶ B: Tension stress forces of “Von Mises” represented on the external surface of the prosthetic components above the articular cavity (fossa), mandibular (below) and C: TMJ fixing screws - Concepts.⁶

Biomechanical and kinetic comparison between TMJ-Concepts and PEEK Prostheses.

The study by⁷ proposed that the mechanical force of the condylar load shapes the eminence of the temporomandibular joint in healthy humans. This was supported by⁸ and Iwasaki et al.⁹ who showed that the eminence's morphology was consistent with the minimization of joint load using numerical models.

To test this hypothesis, an estimate of the individual muscle forces and reaction forces at the TMJ was performed for a given bite force with a form of joint eminence (ASA) variable according to the morphology of the structure.¹⁰ In this case, the inclination of the joint eminence (IEA = ASA) is defined as the angle formed by the AS and the horizontal plane of Frankfort (HF) or any other horizontal plane, such as the occlusal or palatal plane. In adults, the EIA has been reported to range from 30° to 60° in healthy patients (Fig. 6).⁸

Model from the graphical representation in figure (A), as published by Oruba et al.¹¹ and model adapted and modified with graphic representation using the figures as source (B) and (C), as published by Elledge et al.¹² Schematic representation (A) of the method used to measure the angle of inclination of the joint eminence (IEA) according to Csado et al. (2011). B and C: IEA angle represented in TMJ - Concepts prostheses with 41.33° and Prostheses in PEEK with 16.98°, respectively. The program used was ImageJ (Image Processing and Analysis in Java – version 1.8.0).

According to Fig. 7, a parameter analysis was performed on patients with mandibular osteogenic distraction (OD), using a maxillary mandibular model simulator created by.¹⁰ The entry stipulated for the model was a symmetric occlusion force of 191 N (reference in healthy subjects), medium compound between the 2 s premolars perpendicular to the occlusal plane, with angular inclination variation of the articular eminences from 0 to 35° (D/E), considering zero degrees as the IEA wave angle parallel to the occlusal plane.

Fig-7.

The surface represents the average ATM load of individuals before OD as a function of the angles of inclination of the joint eminence, as reported by¹⁰

The results of the studies show that the average joint reaction force of the TMJ as a function of the two angles of eminence (D/E) presented minimum values between 1 and 11°, corresponding to a load on the average TMJ of 94 N. The simulation supports the hypothesis that the differences observed in the reduction of the inclinations of the joint eminences (D/E) are consistent with the minimization of the joint loads in the joint reaction force (Fig. 7).¹⁰

The surface represents the average ATM load of individuals before OD as a function of the angles of inclination of the joint eminence, as reported by.¹⁰

The present values and data, based on the adapted and modified model in Fig. 8, also suggest the importance of the principle of minimizing joint loads in TMJ prosthetic situations; that is, one can assume the condition in which the body adapts its tissues to be able to support (and transfer) a maximum load with a minimum of material installed in the region, known as Wolff's bone adaptation and remodeling law (1892) for the joint tissues exposed to prosthetic components (IWASAKI, 2003). Functionally, there is the best effort mechanism for the new shape and orientation of the mandible condyles, while the bone adapts to accommodate the recent pattern of rotation and joint loading of the prosthetic components (TMJ), so that the muscles of the mandible take on functional control, requested from the joint submitted to surgery. The condition of the relationship of lower inclination angles of the joint eminence with lower intra- and extraosseous pressures (N) seems to be more recommended and favorable to prosthetic materials that offer structural components similar to this mechanism, as observed in Fig. 1 (B and C), and more related to the total PEEK prosthesis (15°) compared to the conventional model of TMJ Concepts (40°).

Fig-8.

Model adapted and modified with graphical representation of the colored lines in the original figure, as published by Zee et al. (2009). The surface represents the comparison of the average TMJ load of individuals using PEEK total prosthesis (orange stripe) and TMJ-Concepts (red stripe) as a function of the angles of inclination of the joint eminence.

Model adapted and modified with graphical representation of the colored lines in the original figure, as published by.¹⁰ The surface represents the comparison of the average TMJ load of individuals using PEEK total prosthesis (orange stripe) and TMJ-Concepts (red stripe) as a function of the angles of inclination of the joint eminence.

According to the physical model in Fig. 9, we can observe that the distance AB between the screws of the mandibular component of the total TMJ prosthesis is greater in the PEEK model (Total = 5) than in the TMJ– Lorenz (Total = 9), showing conditions inversely on the bone surface (blue), a smaller distribution between the points of the bar in the vertical force modules F1 and F2 (screws), as well as a lower torque moment.

Fig-9.

Kinetic representation of the compression forces of the screws in the mandibular prosthetic components of the total TMJ prosthesis models PEEK and TMJ-Concepts.

If we consider an A axis (Fig. 9), for both prostheses, a greater lever arm in PEEK will be determined and therefore, less pressure (Newtons) will be necessary to maintain the component in the mandibular branch (screws) compared to TMJ-Lorenz, with a smaller lever arm and therefore with greater pressure requirement (Newtons) of the screw in the prosthetic component of the branch. In this case, as we can see in the figure below C, we can consider, according to the greater possibility of bone resorption, due to the conformity a greater traction, pressure, compression force for the screws used for stability of the prosthetic part in the element of the mandibular ramus of the TMJ - Lorenz, compared to the PEEK model prosthesis. In this way, this condition determines a higher pressure center in the cervical one-third of the screws, mainly in the first 3 in the condylar neck, which also follows the reaction force of the TMJ in the highest pressure points in Mpa (Megapascal), as observed in Fig. 10 (A). In this case, following the principle of the bone remodeling law due to the greater angle of inclination of the TMJ-Lorenz TMJ eminence, it will be necessary to place more screws in the TMJ-Concepts prosthesis, with a total of nine, compared to the model in question. PEEK, which uses only five screws to stabilize the prosthetic part in the mandibular ramus. This situation corroborates the concept of uniform stress distribution and stable retention, which are prerequisites for the successful clinical application of any prosthesis system. Many clinical complications are related to the concentration of stress in specific regions of the prosthesis, which can trigger catastrophic consequences, such as stress fatigue, resulting in prosthetic fracture.^{12, 13, 14}

Fig-10.

Model adapted and modified with distances and tension stress forces in the screws of the mandibular component (smaller square), as published by Chen et al. (2018), according to the figures published by Wolford (2008) (left) and (right) by Genovesi (2018). Representation (yellow lines) of the distance between the screws of the mandibular components and the articular cavity prostheses TMJ-LORENZ (left) and in the PEEK model (right). The program used was ImageJ (Image Processing and Analysis in Java – version 1.8.0. Measurements are in pixels; these are for length.

Model adapted and modified with distances and tension stress forces in the screws of the mandibular component (smaller square), as published by Chen et al.,⁶ according to the figures published by Wolford¹⁵ (left) and (right) by.¹⁴ Representation (yellow lines) of the distance between the screws of the mandibular components and the articular cavity prostheses TMJ-LORENZ (left) and in the PEEK model (right). The program used was ImageJ (Image Processing and Analysis in Java –version 1.8.0. Measurements are in pixels; these are for length.

The movements of the prosthetic head are directly related to the articulated rotation of the mandible with its center of rotation (CR), positioned in the area of the ramus in the craniocaudal direction. In this case, the closer the CR to the joint cavity prosthesis, the smaller the condyle displacement during the translation movement and the greater the stability in the cavity. (VAN LOON, 1998) (Fig. 11).

Fig-11.

We can see that as the distance increases in relation to the center axis of the system.

As shown in Fig. 12, the PEEK prosthesis (B) shows the RC, around 5 mm below the cavity in the mandibular ramus, while TMJ Concepts has the same RC with a reference of approximately 15 mm, at the lower point of the mandibular ramus (VAN LOON, 1998).

Fig-12.

Rotation center (RC), representation in red, in the mandibular branches of the prosthetic components. In A on TMJ–Concepts located at 15 mm and in B in the PEEK Prostheses is at 5 mm.

Rotation center (RC), representation in red, in the mandibular branches of the prosthetic components. In A on TMJ–Concepts located at 15 mm and in B in the PEEK Prostheses, it is at 5 mm.

This physical quantity (angular torque) is associated with the possibility of rotation around an axis, due to the application of a force on a body. The rotation effect depends on the force intensity and the distance perpendicular to the axis of rotation, as seen in Fig. 13. The torque module is calculated by the product of the force intensity by the distance from the force action line to the axis of rotation; the torque unit in SI is Nm.¹⁶ In the example in Fig. 11, we can see that as the distance increases in relation to the center axis of the system, the rotational force or torque moment decreases, which can also occur inversely proportional to the example.

Fig-13.

Model adapted and modified with graphic representation using the figures as source (A), as in Elledge et al. (2019) and (B), and Genovesi et al. (2018). Representation of torque or angular momentum with different distances of forces perpendicular to the rotational axis. In (A) TMJ-Groningen Total Prosthesis with 49.3 degree (IEA), lever arm with a distance (dist) of 1.04 mm (from the mandibular angle to the distal of the lower third molar) with a force of 170N. In (B) in PEEK total prosthesis with 19.0 degree (IEA), lever arm with a distance (dist) of 1.70 mm (from the mandibular angle to the distal of the lower second premolar) with a force of 98N. The program used was ImageJ (Image Processing and Analysis in Java – version 1.8.0.

Model adapted and modified with graphic representation using the figures as source (A), as in Elledge et al.¹² and (B), and Genovesi et al.¹⁴ Representation of torque or angular momentum with different distances of forces perpendicular to the rotational axis. In (A) TMJ-Groningen Total Prosthesis with 49.3° (IEA), lever arm with a distance (dist) of 1.04 mm (from the mandibular angle to the distal of the lower third molar) with a force of 170 N. In (B) in PEEK total prosthesis with 19.0° (IEA), lever arm with a distance (dist) of 1.70 mm (from the mandibular angle to the distal of the lower second premolar) with a force of 98 N. The program used was ImageJ (Image Processing and Analysis in Java – version 1.8.0.

The force is perpendicular to the path radius; we see that FR = M, the torque modulus exerted by the force F in relation to the center of the circular movement. We have as a result according to Fig. 14, where I = m.R2.

Fig-14.

Sphere of mass m attached to an axis by a wire of length R, under the action of an external force F.

The equation α = M. I, observed in Fig. 14 around the mandibular axes, relates the modulus of the torque M with the angular acceleration α and the quantity I that represents the rotational inertia of the object. The quantity is known as the moment of inertia of the body and its unit in the SI is kg.m². According to Ref. 10; for a lower jaw weighing approximately 0.44 kg, the moments of inertia (healthy individuals) were estimated according to the movements in the 3 axes of rotation with IEA at 40°:

• Z axis = 8.6 kg cm² - Laterality

• Y axis = 2.9 kg cm²- Opening and closing

• Xaxis = 6.1 kg cm² - Inclination (rotation)

According to Koostra et al.,¹ this means that the TMJ requires about three times less muscle torque to accelerate the jaw during the opening and closing movements (Y axis = 2.9 kg cm²) compared to the lateral deviation movements for D/E (Z axis = 8.6 kg cm²) and twice when related to lateral inclination movements with rotation (X axis = 6.1 kg cm²). Considering the surgical placement of a total TMJ prosthesis, with a reduction of approximately 50% in the degrees of angles of inclination of the eminence of the articulation (IEA) of the model in PEEK (19°) compared to TMJ-Groningen (49.3°), considering the sagittal plane as an analysis, we also considerably increased the reference values 2 (two) times greater than the moments of inertia in the 3 axes of rotation of the mandibular movements around 50% (X = 12.2 kg cm²; Y = 5.8 kg cm²; Z = 17.6 kg cm², in favor of PT in PEEK, with increased perpendicular distances and resulting mechanical advantage in the moments of angular torque of the axis mandibular movements (Fig. 15). This expected condition can functionally condition the model of the PEEK Total Prosthesis, greater performance in the mobility and amplitude of the prosthetic joint, as well as less muscular effort to reach the resultant adequate and necessary muscular force of the TMJ articular amplitudes, with the prosthetic components installed. According to Zee (2000), this favorable articular disposition arises for determining angular accelerations, and they can improve both in changes in speed (linear and angular), as well as in changes in position (linear and angular) of biomechanics and mandibular kinetics, in the surgical prognosis of total TMJ prostheses, related to the survey carried out to PT in a PEEK model (see Fig. 15).

Fig-15.

Model adapted and modified with graphical representation with captions (squares) and indicators (arrows) using the figures as source, as published by Koolstra et al. (2002). Six degrees of freedom for jaw movement (model adapted and modified. Dashed lines: main axes. a (linear), accelerations. F (linear), forces. m, mass., angular accelerations. M, torques. I, moments of inertia (healthy individuals), Box (in red) MI moments of inertia (IEA = 40 degrees) similar in TMJ-Concepts, Box (in black/pink) - estimate of MI moments of inertia in PEEK Total Prosthesis.

Model adapted and modified with graphical representation with captions (squares) and indicators (arrows) using the figures as source, as published by Koolstra et al.¹ Six degrees of freedom for jaw movement (model adapted and modified. Dashed lines: main axes. a (linear), accelerations. F (linear), forces. m, mass., angular accelerations. M, torques. I, moments of inertia (healthy individuals), Box (in red) MI moments of inertia (IEA = 40°) similar in TMJ-Concepts, Box (in black/pink) - estimate of MI moments of inertia in PEEK Total Prosthesis.

After reviewing these studies, observing all patients and reading the literature about screw loosening during function, it was concluded that the problem is the wrong design for the glenoid fossa prosthesis and the condyle design.

In 2008, a new design concept for glenoid fossa with a flattened plane and elliptical condyle design was formulated using PEEK LtI 20% Ba. Meaning, unlike a box, it was designed with a 90° angle to the base skull, eliminating the eminence of glenoid fossa prosthesis, allowing the mandible free movement for rotation and translation with the elliptical shape of the condyle, allowing free movements See Fig- 16, Fig- 17, Fig - 18, Fig- 19, Fig-20.

Fig- 16.

PEEK prosthesis design as reported in.¹⁴

Fig- 17.

PEEK prosthesis design as reported in.¹⁴

Fig - 18.

PEEK prosthesis design as reported in.¹⁴

Fig- 19.

TMJ Concepts design, as reported in Elledge et al.¹²

Fig-20.

TMJ Concepts design, as reported in Elledge et al.¹²

After 8 years from the first surgery, the radiograph shows the patient with an opened mouth and the condyle in rotation/translation position. The most important point is that condylar displacement is impossible with this new design of the glenoid fossa and the head of condyle. Also, it can be seen that there is no heterotopic bone formation over the PEEK prosthesis body See Fig- 21, Fig- 22, Fig: 23.

Fig- 21.

Note the prostheses in the apex of the temporal fossa will avoid condyle displacement, trauma at the eminence and no bone resorption around the screws.¹⁴ All TMJ prosthesis systems are in PEEK Lt120%, including the screws.

Fig- 22.

Note the prostheses in the apex of the temporal fossa will avoid condyle displacement, trauma at the eminence and no bone resorption around the screws.¹⁴ All TMJ prosthesis systems are in PEEK Lt120%, including the screws.

Fig: 23.

Note the prostheses in the apex of the temporal fossa will avoid condyle displacement, trauma at the eminence and no bone resorption around the screws.¹⁴ All TMJ prosthesis systems are in PEEK Lt120%, including the screws.

4. Conclusion

After 30 years of TMJ prostheses models, an improved version of the glenoid fossa and condyle design was proposed. It was concluded that it must be flat with 90° to the base skull and with an elliptical-shaped head for the condyle. The material for TMJ prostheses must be light enough to allow all free movements.

It is our hope that these findings inform engineers and oral and maxillofacial surgeons who develop these prostheses, and change the glenoid fossa design to provide better function for patients undergoing TMJ total joint replacement.

A new era in TMJ prostheses design is on the horizon; the glenoid fossa design can be manufactured either in UHWMPE, ceramic, or in PEEK. Our system is in PEEK Lt1 20% Ba.