274

Root and bone response to proximity of miniscrew implantsHyewon Kim and Tae-Woo Kim

50 

INTRODUCTION

Use of miniscrew implants (MIs), particularly inter-radicularly in the alveolar bone in proximity to adjacent roots, carries certain risks and clini-cians need to understand the likely response of roots and surrounding bone and the potential consequences.

ROOT CONTACT WITH MINISCREW IMPLANTS

RISK FACTORS

If injury to a tooth root occurs during MI insertion, loss of tooth vitality, osteosclerosis and dentoalveolar ankylosis may follow.1,2 During ortho-dontic loading, MI failure or migration may occur3 and soft tissue compli-cations such as inflammation or infection may also have subsequent effects on bone and tooth roots.1

EXTENT OF DAMAGE AND PREVENTION METHODS

A good knowledge of the average inter-radicular space in the area of planned implant insertion is essential. Stents and guides that are placed over the adjacent teeth can help to identify the insertion position4–7 and can be used in conjunction with radiography or CT to evaluate the inter-radicular space available and to safely insert the MIs avoiding contact with the adjacent roots.8–10 However, radiography taken after MI insertion to verify root contact has limitations: it only provides a two-dimensional representation of a three-dimensional object, only shows lesions of a certain dimension and cannot indicate the severity of root resorption.11 It has been suggested that the use of self-drilling and self-tapping MIs may decrease the chances of root damage as they improve tactile feedback to the operator during drilling and have less overheating, more bone-to-metal contact and possible decreased mobility.12,13

An experimental study intentionally damaged roots and surrounding structures during MI insertion for 42 MIs in seven beagle dogs and observed that out of the MIs that contacted the root, 7.2% caused direct damage to the periodontal ligament, 19.0% caused damage isolated to the cementum, 26.2% caused damage to the dentin, and 14.2% caused severe damage to the pulp.14 Most of the MIs that failed or were mobile showed bone loss and necrotic tissue in the peri-implant area, which may serve as a stimulus for root resorption. The presence of inflammation increased the damage caused by the MIs.

Failure rates for MIs increase when the implants invade the adjacent roots, being as high as 79.2%, with an average retention period of 16 days.15 Proximity of MIs to dental roots, as seen by radiography and three-dimensional CT, was a major risk factor for MI failure.16

MINISCREW IMPLANT DIAMETER AND CLEARANCE

MIs with a larger diameter increase pressure on the periodontal ligament and both inter-radicular space and MI diameter need to be considered when planning treatment.

Various space recommendations for inter-radicular MIs have been made: 1 mm between the periodontal ligament and the MI,17 at least 5 mm between adjacent roots,18 3.5 mm between adjacent roots for MIs of maximum diameter 1.5 mm19 and a minimum of 2.0 mm between the implant and root surfaces.3

ROOT RESPONSE

ROOT RESORPTION AFTER CONTACT

Root resorption during tooth movement has most often been found where there is overcompression of the periodontal ligament.20

The mechanism of root resorption that occurs after iatrogenic trauma to the root surface, as is the case after contact with a MI, appears to be repaired quite quickly and, when the damage is limited to the periodontal ligament, with no further consequences.21 However, if the cementum layer is mechanically damaged with exposure of the dentin surface, the process of resorption starts. The first changes occur in the periphery of the necrotic tissue where multinuclear cells and cells staining for tartrate-resistant acid phosphatase accumulate.22 These resorbing cells require continuous stimu-lation during phagocytosis, and without further stimulation the process stops spontaneously.

Repair of the affected area occurs through formation of cementum-like tissue within 2 or 3 weeks, depending on the area of the root that is injured. Three different types of root resorption response can be described:23

■ when the MI is in proximity (<1 mm) but bone exists between MI and root (Fig. 50.1A)

■ when the MI thread is away from the root but in contact with the periodontal ligament (Fig. 50.1B,C)

■ when the MI thread touches the root and stays in contact with it without resorption following (Fig. 50.1D,E).

Fig. 50.2 shows a possible sequence of events after root contact with a MI.

Root resorption has been shown to occur indirectly when the MI is inserted in close proximity to the root surface even if a clear width of bone of around 1 mm exists between the root surface and the MI (Fig. 50.1A). This may be a result of pressure from the MI on the alveolar bone, causing bone compression and compression of the periodontal lig-ament. Damage to the periodontal ligament is known to cause root resorption. Macrophages and osteoclasts in viable periodontal ligament will initiate wound healing by removing the damaged tissue. During this activity, bone and cementum can be removed along with necrotic perio-dontal ligament tissue.

Pressure from bleeding into the periodontal ligament may also elicit minor areas of damage to the root surface. This type of edema has been shown to directly affect the arrangement and structure of the extracellular matrix of the periodontal ligament.24 Mechanical stresses to a tooth root may be responsible for vascular flow alterations that trigger cellular degen-eration, leading to hyalinization.25 The first steps of root resorption can be seen as the removal of this hyalinized necrotic tissue.

Root and bone Response to pRoximity of miniscRew implants  275

Healing Versus Non-healing

When any part of the MI thread was left touching the root surface, no definite cementum repair was observed on the damaged root surface (Fig. 50.1D,E).23 After MI contact with the root surface, the possibility of healing through cementum deposition or no healing depends on whether the MI stays in contact with the root surface.

When a MI is left in contact with the root surface, minimal healing occurs in most cases. When a tooth was pushed against the MIs through orthodontic force, there was no sign of repair in the resorption lacunae, but swift repair of tooth surfaces occurred when root contact was discon-tinued, and abundant cellular cementum was deposited.19,21 When a MI is drilled between two roots with large force, the implant may become lodged between the adjacent roots (Fig. 50.1D,E) and this absence of mobility would mean that the pressure between the root and the implant is maintained. Resorption is an active process that requires viable cells in the vicinity of the root surface and the first sign of it occurring is penetra-tion of cells from the periodontal ligament into the mineralized cemen-tum.20 If the periodontal ligament itself is damaged or destroyed, this source of healing cells is lost. The resorbing cells also require continuous stimulation during phagocytosis and the process stops spontaneously if stimulation fails.

When a MI is removed immediately after root contact has been made, relieving pressure on the root, cementum repair occurs on the damaged root surface (Fig. 50.3).

PULP DAMAGE AND RESPONSE

When the MI goes beyond touching the root and penetrates the dentin, much more severe damage can be expected. Dentin tissue is ruptured and rapid formation of osteodentin is observed (Fig. 50.4). Osteodentin is the response of dentin to a severe attack, and consists of tertiary dentin with sparse and irregular tubular patterns with cellular inclusions. This has a similar appearance to bone. Rupturing the pulp tissue seems to have irre-versible consequences and should be avoided.

ANKYLOSIS

Ankylosis may occur after severe root damage from MIs. A patent perio-dontal ligament space is essential to prevent contact between root and

ROOT HEALING

Normal healing has been defined as formation of a cementum layer, normal periodontal ligament attachment and bone regeneration. Abnormal healing is associated with absence of periodontal ligament or bone regeneration and may lead to root ankylosis or pulp invasion.26 The consequences of root damage may include healing without root resorption, surface root resorption that heals with cementoid tissue or replacement resorption leading to ankylosis. It is believed that a favorable environment for root resorption includes destruction of the protective surface covering the min-eralized tissue, presence of vascular connective tissue or an inflammatory stimulus such as bacteria or trauma.27 Damaged roots usually heal unless the damage extends to the pulp.15 Deposition of cementoid tissue starts at the periphery of a damaged area and extends to cover the entire defect over time, with new cementum layers becoming thicker.19 Healing can occur in as short a time as 4 weeks.21 Normal healing, reattachment of the periodontal ligament, regeneration of bone and new cementum layers on exposed dentin are all evident by 6 weeks but if inflammatory infiltration or invasion of the pulp chamber occurs, the normal healing sequence could be interrupted.26

Fig. 50.1  Root resorption with miniscrew implants (MIs). (A) MI in close proximity to root causes resorption (yellow arrows) even though a clear width of bone is present between the MI and the root surface. (B) MI in contact with the periodontal ligament and dentin. (C) Close up of the area in the red box in (B) showing cementum deposition (red arrowheads) on the resorbed root surface. (D) MI thread in contact with root (yellow arrowheads). (E) Close up of red box in (D) showing no cementum repair of the root surface. D, dentin; B, bone; P, pulp; PDL, periodontal ligament. 

A B C D E

P

P

D

D

D

D

D

D

MI MI

MI

MIMI

B BPDL

PDL

PDL

× 12.5 × 12.5 × 12.5× 100 × 100

Fig. 50.2  Possible sequence of events after root contact with a miniscrew implant. 

Root contact

Pulppenetration

Rootapproximation

Osteodentinformation AnkylosisRoot

resorption

Implant immediatelyremoved

Cementumrepair

Dentincontact

Thread in contactwith root

No cementumrepair

No rootresorption

Cementumrepair

276  section ix: efficiency of sKeletal ancHoRaGe and RisK manaGement

Fig. 50.3  Changes seen following immediate removal of a miniscrew implant (MI) after its insertion. (A) Mechanical damage and resorption of the root surface (long yellow arrows) where contact with the MI was made. (B) The area in the red box in (A) showing cementum repair of the resorbed root surface (short red arrows).  D, dentin; PDL, periodontal ligament; B, bone. 

× 40 × 100A B

PDL

PDLD D

B

B

Fig. 50.4  Penetration of the miniscrew implant (MI) into the pulp space. (A) Formation of osteodentin cap (green arrows) around the MI tip. (B) Close-up view of a section showing formation of osteoids (red circles) inside the dentin tissue. P, pulp; D, dentin; PDL, periodontal ligament; B, bone. 

× 12.5 × 100A B

MIMI

PDLD

D

O

O

OO

O

P

bone. If this barrier is severely damaged and bone grows towards the resorbed root, ankylosis may occur. A MI totally perforating a root caused ankylosis in beagle dogs.28 In such severe damage, root resorption and ankylosis were observed on the opposite side to the MI insertion. This may indicate that insertion pressure on one side of the root can induce root resorption and ankylosis on the opposite side. Ankylosis has been observed only in severe injury with displacement of root fragments, and this may be caused by compression of the lamina dura causing by obliteration of the periodontal ligament space.29

An ankylotic response has not been seen when the periodontal ligament space was well maintained even when the pulp was penetrated.23,30 Studies of resorption after replanting of a tooth indicate that the healing responses of the periodontal ligament depend on the extent of the damage to the liga-ment, with up to 2 mm of loss of periodontal ligament being repaired by new attachment without ankylosis.19 With time, minor areas of possible ankylotic spots are expected to be resolved.

BONE RESPONSE

BONE REMODELING AROUND IMPLANT

When a MI is inserted into the alveolar bone, there is a mismatch between the elastic moduli of the two materials at the bone–implant interface, which causes stresses in the bone surrounding the MI and leads to increased bony remodeling at the interface. Masticatory forces may also create strain at the bone surfaces. The bone responds by depositing more bone, which is then remodeled into stronger compact bone with time. However, if there is contact between the MI and the root surface, jiggling forces will be set up at the MI–root contact. It can be speculated that this may decrease mechanical retention and eventually lead to a higher failure rate.15

OSSEOINTEGRATION

Osseointegration can be defined as direct contact between bone and an inert object, with histological evidence suggesting that new woven bone is formed around the inert object and, for a MI, around and within the threads. This newly formed bone is relatively darker than surrounding trabecular bone (Fig. 50.5). The quality and amount of osseointegrated bone around the MIs, as well as other factors such as the degree of inflam-mation, possible excess orthodontic force and the proximity of the MIs to the roots, affect their stability and consequently their failure rates.16

CONCLUSIONS

Every effort should be made to avoid contacting root surfaces when insert-ing MIs inter-radicularly into alveolar bone. If contact is made, resorption of the root surface occurs, with or without cementum healing, depending on specific circumstances. Where excessive force causes penetrative injury to the pulp, irreversible damage occurs with possible ankylosis of the root with the bone. When contact between MI and root is suspected, immediate removal of the MI and reimplantation is recommended, since the removal of the contact will allow healing of the root surface. Care must also be taken when a MI is inserted closer than 1 mm to a root surface, since root resorption may occur even when a thin layer of bone and periodontal liga-ment exists between the MI and the root surface.

Fig. 50.5  Bone response following insertion of a miniscrew implant (MI). (A) Osseointegration of bone and MI (long red arrows). (B) New bone formation next to the MI thread (red N). B, bone. 

× 100 × 100A BMI MI

B B

N

N

N

Root and bone Response to pRoximity of miniscRew implants  277

15. Kang Y, Kim JY, Lee YJ, et al. Stability of mini-screws invading the dental roots and their impact on the paradental tissues in beagles. Angle Orthod 2009;79:248–55.

16. Kuroda S, Yamada K, Deguchi T, et al. Root proximity is a major factor for screw failure in orthodontic anchorage. Am J Orthod Dentofacial Orthop 2007;131(Suppl.):S68–73.

17. Schnelle MA, Beck FM, Jaynes RM, et al. A radiographic evaluation of the availa-bility of bone for placement of miniscrews. Angle Orthod 2004;74:832–7.

18. Gautam P, Valiathan A. Implants for anchorage. Am J Orthod Dentofacial Orthop 2006;129:174; author reply 174.

19. Maino BG, Weiland F, Attanasi A, et al. Root damage and repair after contact with miniscrews. J Clin Orthod 2007;41:762–6.

20. Brudvik P, Rygh P. The initial phase of orthodontic root resorption incident to local compression of the periodontal ligament. Eur J Orthod 1993;15:249–63.

21. Kadioglu O, Buyukyilmaz T, Zachrisson BU, et al. Contact damage to root surfaces of premolars touching miniscrews during orthodontic treatment. Am J Orthod Dento-facial Orthop 2008;134:353–60.

22. Brudvik P, Rygh P. Root resorption beneath the main hyalinized zone. Eur J Orthod 1994;16:249–63.

23. Kim H, Kim TW. Histologic evaluation of root-surface healing after root contact or approximation during placement of mini-implants. Am J Orthod Dentofacial Orthop 2011;139:752–60.

24. Khow F, Goldfaber P. Changes in vasculature of the periodontium associated with tooth movement in Rhesus monkey and the dog. Arch Oral Biol 1970;15:1125–43.

25. Faltin R, Faltin K, Sander FG, et al. Ultrastructure of cementum and periodontal liga-ment after continuous intrusion in humans: a transmission electron microscopy study. Eur J Orthod 2001;23:35–49.

26. Brisceno CE, Rossouw PE, Carrillo R, et al. Healing of the roots and surrounding structures after intentional damage with miniscrew implants. Am J Orthod Dentofacial Orthop 2009;135:292–301.

27. Gold SI, Hasselgren G. Peripheral inflammatory root resorption: a review of the litera-ture with case reports. J Clin Periodontol 1992;19:523–34.

28. Lee YK, Kim JW, Baek SH, et al. Root and bone response to the proximity of a mini-implant under orthodontic loading. Angle Orthod 2010;80:452–8.

29. Renjen RA, Maganzini AL, Rohrer MD, et al. Root and pulp response after intentional injury from miniscrew placement. Am J Orthod Dentofacial Orthop 2009;136: 708–14.

30. Chen YH, Chang HH, Chen YJ, et al. Root contact during insertion of miniscrews for orthodontic anchorage increases the failure rate: an animal study. Clin Oral Implants Res 2008;19:99–106.

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