Anesthesiology

The AMSA injection: A new conceptfor local anesthesia of maxillary teethusing a computer-controlled injection systemMark J, Ftiedtiiati, DDS*/Mark N. Hochtnan,

7ÏIÎJ article describes a new injection technique for the maxillary arch that achieves pulpal anesthesia of thecentral incisor through the second premolar without collateral anesthesia of the face and muscles of expression.This palatal injection can be delivered easily, consistently, and virtually imperceptibly with a recently intro-duced computer-controlled local anesthesia delivery system. The anterior (AMSAj middle superior alveolarblock, is a single-site injection requiring less than one cartridge of anesthetic and is ideaifor maxillary estheticrestorative dentistry because it does nol distort the imile line. A clinical example is aiso presented.(Quintessence Int 1998;29:297-303)

Key words: anterior and middle .superior alveolar neurovascular bundles, maxillary local anesthesia

Clinical relevance

The described anesthesia system provides dentistswith a contemporary alternative to the traditionallocal anesthetic syringe and represents a practicaland efficacious concept for local anesthesia of max-illary teeth.

Dentists have traditionally anesthetized teeth in themaxillaty arch with a supraperiosteal injection in

the mucobuccal fold in proximity to the apices of theteeth to be anesthetized,' A recently introducedcomputer-controlled local anesthesia delivery systetnpetmits achievement of predictable and profound pulpal

•Clinical Professor of Reswralive Denlistry. tjnivemly of SouthernCalifornia School of Denlistry, Los Angeles, California; ClinicalDireclor, Milestone Scientific. Livingston, New Jersey

'Clinical Assistant Professor, Department of Postgraduate Periodonties,State tJniversity of New York at Stony Brook, School of Dental Medi-cine; Associate Clinical Director, Milestone Scientific, Livingston,New Jersey,

•'CliS

[New jersey,ReprinI t^quesls: Dr Mark J. Friedman, Center for Dental Aesthetics,16830 Ventura Blvd, Suite 258, Encino. California 91436. E-mail:,^ f n p. i m n (fi hsc, use, edumfrieiima@hsc,usc,edu

anesthesia of multiple maxillary teeth from a singleinjection, using a minimal dosage of anesthetic and withno collateral anesthesia of overlying facial structures.This paper describes the clinical technique for anes-thetizing the anterior and middle superior alveolar(AMSA) neurovascular bundles from a single, virtuallyimperceptible palatal injection via this computer-controlled device,- This new and highly predictableblock injection simplifies anesthesia of maxillary teethand has a significant positive impact on clinical proce-dures and patient comfort,̂

History

A Supraperiosteal injection in the mucobuccal fold isthe most commonly utilized route of administration toachieve local anesthesia of maxillary teeth,-» This injec-tion is referred to as an intiltration or field block andwas first described by William Halsted in the late1800s.' It is a convenient, safe, and effective means ofpain control for a variety of procedures performed onmaxillary teetb and associated soft and hard tissues oftbe region," Anesthetic solution diffuses from theinjection site, penetrating throngh the soft tissues,periosteum, and porous maxillary bone, and resulting inanesthesia of the radicnlar nerve fibers of the teeth inproximity to tbe injection site/

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Fig 1 a A 27-year-old patient with lips in repose to allow for accu-rate esthetic assessment of the six porcelain veneer restorations(hat have been placed on the maxillary anierior teeth.

Fig l b Same palient atter a traditional infiltration of 0.6 mL oí lido-caine with 1:100,000 epinephnne in the mucobuccal fold adjacentto (he left lateral incisor demonstrates temporary lip distortion.

Multiple factors play a role in whether adequate pul-pal anesthesia is achieved from a maxillary infthration.They include density and thickness of the bone in thearea, access to the anatomy, length of ihe tooth root(s) tobe anesthetized, type and dosage of tbe anestheticemployed, and ihe patient's subjective paiti thresholdand re.sponse to painful stimuli,*-̂ Since it ts not uncom-mon for variations in anatomy to reduce the success ofan infiltration, multiple needle penetrations may be nec-essary to ensure that an adequate volnme of anestheticsolution is deposited to achieve the desired level of paincontrol for the targeted teetb,'' it is reasonable to predictthat a maxillary field block, or infiltration, anterior tothe first molar will result in concomitant anesthesia ofthe lip, surface of the face, and mu.scles of expression,'"The degree of collateral anesthesia often observed sug-gests that a significant portion of the anesthetic bolus,which is intended for the neutovascular bundles of theteeth at the site of the injection, dissipates into the over-lying soft tissues," Thus, only a portion of the anestheticvolume administered actually reaches the intended tar-get site. The remaining superfluous solution affects fis-sues that ideally should remain unanesthetized.

Maxillary teeth can also be anesthetized with a varietyof block injecfions (eg, infraorhiial, posterior-superioralveolar, second division, etc), as well as intraligamcn-tary (PDL) or intraosseous injections,'- However, the lit-erattire does not descrihe a single injection site thatwould produce pulpal anesthesia to a majority of themaxillary teeth and do so without collateral anesthesia ofthe face, lip, and muscles of expression.

Site-specific anesthesia

A Kite-specific injection that would accurately target theneurovascular bundles associated with tbe teetb, while

minimizing any effect to overlying soft fissues, wouldpositively impact clinical techniques and patient com-fort. In the anterior region of the maxilla, esthetic para-meters are often of critical concern, Lip-to-teeth rela-tionship assessments, considered vital to the quality ofthe finished results, cannot be adequately performed ifthe lip or muscles of expression are inadvetiently anes-thetized^' (Figs la atid lb). Therefore, a site-specificmaxillary injection would greatly enhance the operator'sability to make accurate esthetic assessments while attbe .same time providing an adequate level of pain con-trol. The patient would not experience the annoying sen-sation of facial numbness and the accompanying loss ofmuscle control associated with traditional non—site-specific injection techniques. Furthermore, if tbe major-ity of the administered anesthetic solution specificallytargeted the teeth and adjacent gingival tissues, anes-thetic dosages might he reduced without sacrificing theefficacy and duration of the dental anesthesia.

The ideal maxillar\' injection would produce a rapidonset of profound pulpal anesthesia for multiple teethfrom a single needle penetration. It would not produceunnecessary collateral anesthesia, and it would onlyrequire a minimum dosage of anesthetic solution to beeffecfive. It would be easy to administer without risk toany vital structures and would be virtually impercept-ible during administrafion, making it of itnmense bene-fit to both the patient and the denfist,

A computer-controlled injection system

The Wand Local Anesthesia System (Milestone Sci-entific) is a computer-controlled injection device thesize of a paperback book. It accommodates a conven-tional local anesthetic cartridge that is linked by micro-tubing to a disposable, lightweight, penlike handle with

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a Luer lock needle attached (Fig 2). The computer-con-trolled system is activated by a foot control that auto-mates the delivery of local anesthetic at precise pressureand volutiie ratios, resulting in an effective and comfort-able injection. This eliminates the variahility of thethumb-operated plunger used in a manual syringe.

The autotnation provided by computer control sus-tains a constant pressure and volume ratio of anestheticfluid regardless of variations in tissue resistance. Thus,if the anesthetic solution is deposited into a tissue ofresilience such as the palate, the computer drive com-pensates to ensure a steady and continuous rate of flow.Conversely, if the encountered resistance is reduced,plunger pressure is automatically adjusted.

Patients who have experienced an injection with thissystem report a virtually imperceptible needle penetra-tion followed by a sensation of mild pressure.- Themanufacturer suggests that needle penetration and ad-vancement he done very slowly to allow for anestheticto precede the path of the needle. One can speculate thatmaintaining an anesthetic pathway and an ideal rate offlow are the major factors responsible for achievingcomfortable injections with the system.'•"

The AMSA nerve block

The fact that pulpal anesthesia can be accomplishedfrom a palatal injection is well documented.' '-'* Duringprototype development of the Wand Local AnesthesiaSystem, an anecdotal finding (R. Spinello, York, PA)suggested that comfortable palatal pulpal anesthesia wasfeasible. Clinical investigation determined that theadministration of 0.6 to 0.9 mL of local anesthetic,deposited at a specific single palatal site, produces pro-found pulpal anesthesia of the AMSA neurovascularhundles- (Fig 3). In addition, the palatal soft tissues ofthe region are anesthetized. Because of the palatal ap-proach, this injection does not affect sensory fibers tothe lip and face or alter the activity of the muscles offacial expression. Within 2 minutes of administration ofthis hlock injection, pulpal anesthesia extending fromthe central incisor to the second premolar is achieved.The palatal tissues extending from the midpalate to thefree gingiva and from the central incisor to the firstmolar are also profoundly anesthetized from the injec-tion. The duration of anesthesia is reported to be from45 to 90 minutes. Hemostasis is also achieved in thesame region, and some crossover hemostasis and anes-thesia effect has been observed on the buccal aspect ofthe aforementioned teeth. This observation may be due.in part, to the efficient suffusion of anesthetic into themedullary bone that the controlled flow rate creates. Asufficient concentration of anesthetic solution deposited

Fig 2 The Wand computer-controiied iooai anesthetic deiiverysystem consists ot a microprocessor/drive unit, foot controi, andiightweigiit piastio disposable handpieoe.

in proximity to the major nenrovascular bundles of theanterior and middle superior alveolar plexus mightaccount for the marked hemostasis and profound anes-thesia that can be developed from just 0.6 mL of lido-caine with 1:100,000 concentration of vasopressorinjected inio a single location.

It is speculated that due to the resilience of the palataltissues, the majority of the anesthetic solution that isinjected under precisely controlled pressure and flowrate reaches the tmderlying bone and neurovascularanatomy instead of being dissipated into the surroundingsoft tissues. Based on chnical observations, it is conjec-tured that this injection is intraosseous in nature.

Traditionally, injections in the palate have been as.so-ciated with a significant degree of discomfort."' The lit-erature is replete with articles describing ways tominimize the discomfort associated with palatal injec-tions.""-' The ability of the computer-assisted device todeliver anesthetic at a high pressure, but a slow rate ofvolume flow, is the core technology of the system. Theprimary explanation for the lack of discomfort reportedwith these injections is the separation of anesthetic flowrate from pressure.'•• With a manual syringe, these pres-sure and volume parameters cannot be preciselycontrolled and they are directly linked." In manual de-livery, an increase in pressure is automatically accompa-nied with an increase in volume. Any distention ofpalatal tissues results in significant discomfort for thepatient. If the operator attempts to slowly inject therequired 0.6 to 0.9 mL of anesthetic solution manuallyto achieve an AMSA block, muscle fatigue and physicallimits make the procedure difficult. In addition, the pre-cise pressure gradient that must be achieved and main-tained for anesthetic to efficiently suffuse into thedeeper anatomic structures without eliciting pain cannot

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Fig 3 Sagiftai section diagram depicting the distribution of the superior aiveolar nerve ASA = ante-rior superior aiveolar nerve: MSA - middle superior alveoiar neive: PSA - posterior superior aiveoiar

Fig 4 Paiatai diagram depicting the injection site fcr the AMSAbiocii injection. This site is considérée) to be approximately 1 cmin diameter and, therefore, not an exact point but rather a zone.

Figures 2. 3, 4, 5, 6, 8a, 8b, and 8o are reprinted with permissionfrom The Compendium of Continuing Education in DentistryFriedman MJ. iHoohman MN: A 21st century computerized injec-tion system for iocal pain controi. Compend Contin Educ Dent18(10). 995-1004, 1997.

be reproduced manually- A recently published clinicalstudy evaluating the subjective pain response from acomputer-assisted palatal injection with a controlledflow rate demonstrated that 4S of 50 dentists reportedthis injection caused minimal or no pain-'''

The AMSA injection site is located at a point thatbisects the maxillary first and second premolars and ismidway between the crest of the free gingival marginand the midpalatine suture (Fig 4). The use of a 30-gange extra-short needle is ideal for the administrationof the AMSA block. Topical anesthetic can be appliedto the injection site. However, patients report that evenin the absence of topical anesthetic, the injecfion experi-ence is virtually imperceptible.

The needle is oriented at a 45-degree angle with thebevel facing the palatal tissue, as indicated in a tradi-tional palatal injection technique.--' As the bevel con-tacts the fissue, the foot switch is activated on the slow-rate position to ensure a positive flow of anesthetic atthe moment of needle penetration (Figs 5 and 6)- Be-cause the anesthetic flow rate is precisely controlledautomatically, the operator need only concentrate onthe accurate and delicate placement of the needle,which is greaUy enhanced by a pen grasp of the ultra-light handpiece.

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Fig 5 Palatal view of a 30-gaüge extra-short needle in positionfor palatal AMSA penetration with anesthetic flow rate initiated. Notopical anesthetic was employed

Fig 6 Needle is advanced as slowly as possible to ertsure lhecreation of an anesthetic pathway.

Patients who have experienced an AlVlSA injectiondescribe a slight sensation upon needle penetration andthereafter a mild feehng of pressure that is not unpleas-ant. Seconds after needle penetration, a definite bianch-ing of the soft tissue surrounding the injection site isobserved, indicating that the anesthetic is actively suf-fusing through the connective tissues, periosteum, andcancellous bone. The foot contro] has a ]ow- and a high-rate position; in tissues of resistance such as the paiatc,the injection rate is maintained on the low position.Approximately 60 to 90 seconds may be required toadminister the 0.6 to 0.9 mL of anesthetic solution nec-essary to achieve intrapulpa! anesthesia of the centralincisor through the second premolar for approximately45 to 90 minutes. Additiona] anesthesia can be easiiytitrated in the area as required.

It is imperative that the operator visually monitor thelevel of tissue blanching. If excessive blanching occurs,indicated by a loss of all pink color to the tissues, amomentary pause is indicated to allow the anesthetic todissipate and blood supply to return (Fig 7). In theunlikely event that marked ischemia develops, an ulcer-ative lesion may result within 24 to 48 hours of theinjection. The lesion will resolve itself in 7 to 14 days.

Clinical example

A 46-year-old female patient sought elective estheticrestorative dentistry for correction of multiple, inter-proxima!, discolored composite restorations on themaxillary anterior teeth. In addition, she wished to havethe shade of her teeth lightened. The treatment plan wasto restore the six maxillary anterior teeth with porcelainveneers to mask existing composite restorations and

Fig 7 View ol the palate of a 35-year-olú patient 2 minutes afterbilateral AMSA blocks were administered. Note the signifioantpalatal blanching.

increase the value to approximately a Vita A-1 shade(Fig 8a). The patient was an interior designer by profes-sion and reported having a discriminating perceptionrelative to color and shape, Pretreatment di.';cussionstook place to review the importance of proper alignmentand shape of the tinal restorations with the natural lipline. The patient agreed that this was a critical elementto the final outcome and expressed a strong desire thatevery effoit be made to achieve an ideal result.

At the initial operative appointment the patient wasanesthetized with bilateral AMSA injections. Eachinjection was delivered with The Wand LocalAnesthesia System utilizing a 30-gauge extra-shortneedle. At each injection site 0.9 mL of lidocaine with1:100,000 epinephrine was administered without the useof topical anesthetic. The patient reported that the inser-tion of the needle was barely perceptible and was fol-

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Fig 8a Preoperative conditions of a 46-year-old woman with mul-tipie interproximai resin composite restorations. The treatmentpian caileö for her to receive six porceiain veneer restorations.

Fig 8b The paliecil jeceivdd tjilaLeral AMSA biock injeclions. inspite of compiete puipai anesthesia, the lips and muscles ofexpression remain unaffected and undistorted.

Fig 8c immediateiy after ff>.e cementation of the porceiain veneerrestcrations, the patient is abie lo place lipstick and smile nor-maliy even though ihe teeth are compieteiy anestiietized.

lowed by a mild sensation of pressure. Each injectionrequired approximately 90 seconds to administer.Within 2 minutes the teeth were presumed to be fullyanesthetized, as evidenced by the significant palatalblanching extending from the central incisor to the ,sec-ond premolar. Routine porcelain veneer preparationswere performed with a fine No, 62 diamond (LascoProducts) and high-speed turbine (KaVo) witbout com-plication. During the 20 minutes required to prepare thesix anterior teeth, the patient did not exhihit any behav-ior suggesting that she experienced any iinpleasant sen-sations, and no additional anesthetic was required. Theoutline form and incisai reduction of the completedpreparations was referenced to the lips in repose toensure that adequate tooth structure had heen removed(Fig 8b), The master impression was made with

Impregum Pentamix impression material (ESPE), Thepatient was instructed to wear a maxillary bleach splintas a provisional restoration and was dismissed. Thepatient was contacted on the evening of the appointmentand reported that the anesthesia had dissipated withoutcomplication and that there was no residual discomfortat the site of the injections.

At the cementation appointment, the patient reportedthermal sensitivity. It was decided that a bilateralAMSA would again be administered as a means of paincontrol during the try-in and bonding pbase of this ap-pointment. One cartridge of lidocaine with 1:100,000epinephrine was administered with 0,9 mL being deliv-ered at each palatal landmark. The injection proceededwithout incident and resulted in rapid and profound pul-pal anesthesia. The porcelain veneers were temporarilyplaced on the teeth with water and evaluated in relation-ship to the lips in repose. Minor adjustments wererequired prior to bonding the veneers in place. SingleBond dental adhesive and clear Indirect Luting Cement(3M Dental), were used to adhere the veneers to theetched enamel stirfaces. After photocuring, excess lut-ing material was removed with a No, 12 scalpel blade(Bard Parker), Incisai refinements were made with a104/065 fine diamond disk (Brasseler), and final polish-ing was done with silicone abrasive wheels (DentalDistributors), The patient was pleased with the estheticand functional results of the finished restorations. Be-cause the AMSA injections were utilized, no errorswere made in the alignment of the veneer restorationswith the lips, in spite of tbe fact that tbe patient wascompletely anesthetized during the preparation andcementation phases of treatment (Fig 8c),

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Summary and conclusions

A newly introduced computer-controlled local anesthe-sia system provides dentists with a contetnporary alter-native to the traditional local anesthetic syringe. Thecomputer drive and the disposahle, ultralight handpieceenhances traditional injection techniques and tnakesnew site-specific injections possible. The system con-trols the volume and pressure ratios of anesthefic solu-tion, resulting iti a precision fiow rate, A drop of anes-theUc solution precedes the needle even in tissues ofresilience such as the palate, creaUng an anesthetic path-way for a virtually imperceptible injection without theuse of topical anesthetic.

Because palatal injections can now be administeredwith operator ease and complete patient cotnfort, a newpalatal-approach anterior, tniddle superior alveolarblock injection has been described. Profound pulpalanestbesia of the maxillary central incisor through thesecond premolar and the palatal tissue in the region canbe obtained from 0,6 to 0,9 mL of anesthefic soltitionwithout any numbness of the lips, face, or muscles ofexpression. The AMSA injection represents a practicaland efficacious new concept for local anesthesia ofmaxillary teetb, Otber site-specific local anestheticinjections may also be discovered using this computer-controlled technology.

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