Concept: Trigger Point Injection

Source: Travell & Simons' Myofascial Pain and Dysfunction: The Trigger Point Manual, Vol. 1, 2nd Ed. Chapter 3: Apropos of All Muscles, Section 13, pp. 150–166.

This page is a standalone high-risk clinical reference. For non-invasive treatment, see Concept: Apropos of All Muscles — Treatment. For assessment, see Concept: Apropos of All Muscles — Assessment.

⛔ HIGH CLINICAL RISK — READ BEFORE PROCEEDING

Trigger point injection carries significant clinical risks including pneumothorax, vascular injection, nerve damage, infection, and severe myotoxicity if inappropriate agents or concentrations are used. This page should be read in full before any injection is attempted. Key safety requirements are highlighted throughout.

The following are absolute contraindications to TrP injection:

Patients on anticoagulation therapy.
If the patient has taken aspirin within 3 days of injection.
Tobacco smokers, unless they have stopped smoking and have taken at least 500 mg of timed-release Vitamin C for 3 days prior to injection.
Patients who have an inordinate fear of needles.

One well-performed injection can fully inactivate a TrP immediately. Success depends strongly on the accuracy of the clinician's aim — on the precision with which the TrP was localised and on the skill of the clinician.

Definitions

When reporting TrP injections, for each injection the clinician should specify the muscle injected, and whether it was a central TrP in the endplate zone or an attachment TrP that was injected.

The number of injections should be counted in terms of the number of TrP sites injected, not the number of times some solution has been deposited within one TrP site. One TrP site has a highly variable number of active loci that must be inactivated and all of the loci in one TrP can be needled or injected with one skin penetration.

Why Inject?

The decision whether to treat TrPs by manual methods or by injection depends strongly on the training and skill of the practitioner. Ideally, both approaches should be equally available to the patient and used when indicated.

Manual methods are specifically indicated when:

The TrP is acute.
The goal is to train the patient in effective methods of self-management of the pain and dysfunction.
The patient is severely needle-shy.
The central TrPs in the middle of the muscle belly are not accessible to injection (e.g., iliacus and psoas muscles).

Injection is indicated when:

A few TrPs remain that are unresponsive to manual methods.
Skilled manual TrP therapy is not available.
There are only a few relatively acute TrPs and treatment time is severely limited.
The patient has hyperuricaemia and symptoms of gout.
Injections can be helpful when the muscle cannot be stretched for mechanical reasons, or when stretch should be limited because of hypermobility.

It is a serious mistake to judge the efficacy of TrP treatment by manual methods or by injection if the practitioner was not both well trained AND experienced in the techniques used for the muscles reportedly treated.

Patients with both fibromyalgia and myofascial TrPs are much more sensitive to painful therapeutic techniques (which, for them, are likely to be counterproductive) than are patients with TrPs but without fibromyalgia. Patients with both conditions respond to TrP injections, but they do not respond as well as patients with only myofascial TrPs.

What to Inject?

Dry Needling vs. Injection

Dry needling is as effective as injection of an anaesthetic for relief of TrP symptoms, IF the needle elicits LTRs, which occur when the needle encounters active loci of the TrP. Conversely, if no LTR occurs, dry needling and injection of nontoxic anaesthetics are equally ineffective. Postinjection soreness is more likely to occur with dry needling, is more severe, and is of longer duration following dry needling.

The critical therapeutic factor in both cases is mechanical disruption by the needle. This is consistent with the understanding that disruption of the TrP contraction knots terminates the basis for a local energy crisis and its sensitisation of nearby nerves.

Procaine (Recommended)

Dr. Travell recommended a procaine concentration of 0.5% in physiological saline because greater concentrations showed no additional increase in anaesthetic effect, and no serious harm is done by using a 1% solution. However, the higher concentration has no known advantage; an accidental nerve block will last longer and the higher concentration has a correspondingly greater toxicity to the muscle and systemically.

The maximum amount of procaine that should be injected at one time is 1 g, which would permit the injection of 100 ml of 1% procaine. Since only a few tenths of a millilitre of local anaesthetic are deposited at a time within a TrP, it rarely is necessary to inject a total of more than 20 ml at one visit.

Procaine is the least myotoxic among the local anaesthetics that are commonly injected. Pain sensation following nerve block reappeared in 19 minutes after 1% procaine, and in 40 minutes after 1% lidocaine. Procaine and chloroprocaine have the lowest systemic toxicity of the commonly used local anaesthetics.

Note: 0.5% strength of procaine is not commercially available. The clinician must dilute 2% procaine solution to 0.5% with isotonic saline solution (which is not so irritating to the muscles as sodium bisulphite and has local anaesthetic properties of its own).

A bacteriostatic agent commonly added to procaine is sodium bisulphite, which can be irritating and contribute to postinjection soreness. This effect can be reduced by diluting 2% procaine solution to 0.5% with isotonic saline solution.

Epinephrine is NEVER recommended for the injection of TrPs.

Lidocaine

A 1% solution of lidocaine is commonly used successfully instead of procaine. Lidocaine is not merely a longer-acting form of procaine — these two agents have different intermediate chains and different aromatic residues. Unlike other amide local anaesthetics, lidocaine is an aminoacyl amide. Lidocaine is more effective in a neutral solution; procaine is more potent in an alkaline solution. Procaine is hydrolysed in the blood stream; lidocaine is removed from tissues through solubility in fat and is metabolised primarily in the liver.

Lidocaine is clearly more myotoxic than procaine, and lidocaine is clearly more myotoxic. Myotoxicity, particularly of the longer-acting anaesthetics, is strongly related to the concentration injected. It is unlikely that solutions stronger than 0.5% are any more effective when injecting TrPs.

Isotonic Saline

Sola and Kuitert treated a series of 100 patients with myofascial TrPs by injecting isotonic saline (which also contained a bacteriostatic agent) with multiple needle penetrations in a fan-wise pattern. Frost et al. did a controlled, double-blind comparison between isotonic saline and a long-acting anaesthetic, mepivacaine. Using TrP criteria for the precise localisation of the injection, they found that the saline afforded equal, or more pain relief than injection of the same volume of 0.5% mepivacaine, which is myotoxic.

Corticosteroids

Corticosteroids are potent anti-inflammatory agents and therefore appropriate for the treatment of conditions characterised by an inflammatory reaction. The pathophysiology of a central TrP in the muscle endplate zone involves sensitisation of nociceptors secondary to a local energy crisis. The clinical experience to date indicates that nonsteroidal anti-inflammatory drugs are not effective in reducing the nerve sensitisation in central TrPs. Addition of injected steroids here apparently offers no advantage.

On the other hand, the nerve sensitisation at attachment TrPs is the result of chronic mechanical stress that may produce aspects of an inflammatory reaction that would be responsive to corticosteroids. The definitive treatment for attachment TrPs is inactivation of the central TrPs responsible for them. However, prompt reduction in the tenderness and irritability of the region of enthesopathy at the attachment TrP is therapeutically beneficial to the patient's comfort and most likely helps to reduce the irritability of the corresponding central TrP.

Use of long-acting (deposit) steroids is NOT recommended for the injection of TrPs. Such preparations may be, by themselves, destructive to muscle fibres. It can increase the danger of muscle and connective tissue tears. Long-acting steroids are generally irritating to nerves and can produce complications. Use of deposit steroids enhances the danger of a systemic Cushing-oid reaction with repeated injections. Repeated use of corticosteroid injections followed by ultrasound has been reported to produce depression and atrophy of skin and subcutaneous tissue that resulted in the need for surgical repair.

Botulinum Toxin A

Botulinum toxin type A (BTA) binds irreversibly to presynaptic cholinergic nerve terminals, which includes the terminals of motor nerves supplying skeletal muscle-fibre endplates. Once internalized, BTA blocks exocytosis of the neurotransmitter acetylcholine (ACh), which permanently terminates any neurogenic muscle fibre contractions mediated by the affected endplates. The skeletal muscle that has been thus chemically denervated remains paralysed until a motor nerve sprouts new axons and forms new synaptic contact to reestablish a functional neuromuscular junction for each of the affected muscle fibres. Axon sprouting and muscle fibre reinnervation terminate the clinical toxic effect of BTA, usually in 2–6 months.

Botulinum toxin A injection for treatment of myofascial TrPs has been reported by several authors to be clinically effective. BTA injection should be specific to motor endplate dysfunction and should be injected where a systematic search reveals endplate potentials. Since the spontaneous endplate potentials are highly correlated with TrPs, this is an ideal way to determine exactly where to inject the BTA for maximum TrP effectiveness.

It is important when using BTA to inject the minimum amount necessary and only in the TrP, since BTA destroys normal and dysfunctional TrP endplates alike.

There is NO basis for injecting botulinum toxin A into attachment TrPs.

Myotoxicity

Procaine and lidocaine are the least myotoxic of the local anaesthetics that are commonly injected intramuscularly, and lidocaine is clearly more myotoxic than procaine. Myotoxicity, particularly of the longer-acting anaesthetics, is strongly related to the concentration injected. Solutions stronger than 1% become increasingly and significantly myotoxic. Longer-acting anaesthetics tend to be more myotoxic than shorter-acting ones.

Epinephrine severely increases myotoxicity without conferring any appreciable clinical advantage when injecting TrPs.

Summary of myotoxicity by agent (least to most): procaine (0.5%) → lidocaine (0.5%) → lidocaine (1%) → lidocaine (2%) → longer-acting agents → any agent + epinephrine.

How to Inject?

Patient Positioning

The patient should be recumbent for ANY injection, to avoid psychogenic syncope and falling to the floor. When the patient sits in a chair (or worse is standing), injections can be hazardous in susceptible individuals. Recumbency also greatly facilitates locating the TrP, since the patient is more comfortable and relaxed.

Vitamin C and Aspirin

The increased capillary fragility characteristic of a low serum vitamin C level can cause excessive bleeding in muscles injected for TrPs. Capillary haemorrhage augments postinjection soreness and leads to unsightly ecchymoses. A frequent source of increased bleeding due to low vitamin C is tobacco. Mega-dose vitamin C therapy daily for 1 week should correct this deficiency. At least 500 mg of timed-release vitamin C three times daily is recommended for a minimum of 3 days prior to injection of TrPs.

A daily dose of aspirin increases the susceptibility to bleeding. The patient should take no aspirin for 3 days before TrP injection or needling.

Needle Selection

For the techniques recommended here, the needle length must be sufficient to reach the contraction knots in the TrP to disrupt them. The needle diameter is more a matter of personal preference and skillfulness developed through practice, except in locations where pneumothorax is a serious consideration (precision technique with maximum control is then needed).

Table 3.5: Corresponding Needle Sizes in Metric (mm) and English (Gauge) Systems
Metric (mm)	Gauge Size
0.30	30
0.40	27
0.45	26
0.50	25
0.60	23
0.70	22
0.80	21
0.90	20
1.1	19
1.6	16

22-gauge, 3.8-cm (1.5-in) needle — usually suitable for most superficial muscles.
25-gauge, 3.8-cm (1.5-in) needle — for hyperalgesic patients; may cause less discomfort but will not provide the clear "feel" of structures being penetrated and is more likely to be deflected by the dense contraction knots.
27-gauge, 3.8-cm (1.5-in) needle — even more flexible; tip more likely to be deflected by contraction knots and provides less tactile feedback for precision injection.
21-gauge, 5-cm (2-in) needle — generally long enough for TrPs in the deepest muscles such as gluteus maximus or paraspinal muscles, in non-obese persons.
Obese patients injecting deep muscles may occasionally require a 8.9-cm (3.5-in) needle.

NEVER insert the needle solidly to its hub because the needle is most likely to break off at the hub. The needle should never be inserted solidly to its hub; always ensure some additional depth of penetration can be obtained safely by indenting the skin with a finger beside the needle.

A needle with a burr at the tip must not be used — when the tip of a disposable needle contacts bone, the impact frequently curls the tip to produce a "fishhook" burr that feels "scratchy" and drags as the needle is drawn through tissues; it causes unnecessary bleeding. This is especially important to avoid when injecting TrPs in muscles like the scaleni, which lie near nerve trunks.

Cleansing

An aseptic technique is ensured by careful cleansing of the skin with a suitable antiseptic, avoiding areas suggestive of local infection, and by using uncontaminated sterile solutions and properly sterilised or disposable needles and syringes. The clinician must ensure the skin is dry before injection; if the needle is inserted through wet alcohol it produces a stinging sensation as the needle carries some of it into the skin.

Painless Skin Penetration

Most patients are terribly afraid of the skin pain caused by needle penetration. The skin pain is avoidable with the use of cold anaesthesia, but their fear is not avoidable.

Options for painless skin penetration:

Vapocoolant spray — provides simple answer of cold anaesthesia; effectively blocks nerve conduction when the skin temperature falls to 10°C (50°F). After carefully disinfecting the skin with alcohol, one applies the vapocoolant spray from a distance of about 45 cm (18 in) for 5 or 6 sec (just short of frosting), and then introduces the needle quickly after the stream of spray stops.
Insert needle very quickly through the skin with a flick of the wrist — recommended ONLY for initial penetration with a fresh disposable needle.
Place the skin under marked tension so that the additional tension of needle penetration is hardly noticeable.
Increase skin tension by pinching a fold of the skin between the thumb and fingers — recommended ONLY for initial penetration.

Preinjection Blocks

It is now well established that even brief exposure to considerable pain can cause long-lasting neuroplastic changes in the spinal cord that tend to enhance pain. For patients who are particularly pain-sensitive, or who have found the pain produced by needle encounter with TrPs seriously distressing, a preinjection block can be helpful. This is a newly introduced procedure and must be adopted with due caution. It is described in detail by Fischer. Two methods: (1) diffuse infiltration of local anaesthetic proximal to the area to be injected; (2) infiltration of the entire TrP area with local anaesthetic before needling individual active loci. Use 0.5% procaine because of its lower myotoxicity, its relative innocuousness if a vessel were accidently injected.

Precision Injection Technique

The following precision technique is the one presented in the first edition of this volume. It is a basic technique applicable to central TrPs in any muscle location that can be reached with a needle.

Step 1: Localise the TrP

Localisation of a TrP is done mainly by the practitioner's sense of feel, assisted by patient expressions of pain and by visual observation of LTRs. Identify by gentle palpation the taut band in the muscle, then for a firmer nodule in the taut band, and then for exquisite spot tenderness of the nodule (the TrP). The TrP is also the most responsive spot for eliciting LTRs by snapping palpation or by needle insertion.

The three methods of palpation (flat palpation, pincer palpation, and deep palpation) are fully described in Section 9 of this chapter (see Concept: Apropos of All Muscles — Assessment).

More precisely the TrP is localised, the more satisfactory will be the injection.

When flat palpation is used: position can be confirmed precisely by pushing the nodular TrP back and forth between two fingers (Fig. 3.15A and B). The TrP can then be fixed for injection by pinning it down midway between the finger tips (Fig. 3.15C).

When pincer palpation is used: the TrP is held tightly between the thumb and fingertips. The nodule lies in the endplate zone near the centre of the muscle fibres. For injection, the TrP is held tightly between the thumb and fingertips.

Step 2: Position and Tension the Muscle

For injecting central TrPs, employing any of the three methods of palpation, the muscle fibres of the taut band are placed on sufficient stretch to take up any slack but not enough stretch to cause additional pain. This tautness is necessary to help hold the TrP in position.

Step 3: Skin Cleansing and Anaesthesia

Cleanse the skin. Apply vapocoolant spray if needed for painless needle penetration.

Step 4: Insert the Needle

Warn the patient beforehand that successful needle contact with a TrP may produce a flash of distant pain and likely will cause the muscle to twitch.

For flat palpation injection: the needle is inserted between the fingers that have located the TrP, 1 to 2 cm away from the TrP so that the needle can approach it at about 30° to the skin. Adequate tension of the muscle fibres is required to penetrate the TrP.

For deep muscles: the position of the finger on the skin and the precise direction of maximum tenderness are carefully noted. The needle is then inserted exactly where the finger was and directed in precisely the same direction as the maximum tenderness.

Step 5: Needle the TrP — Confirm with LTR

The needle should explore both the deep and superficial fibres of the muscle. The dense contraction knots in a TrP often feel to the practitioner as if the needle tip has encountered hard rubber that is resistant to penetration and tends to slide to one side. Using the needle as a probe, the TrP sometimes feels like a dense globule, 2–3 mm in diameter; resistance to penetration helps to identify it.

If an LTR and referred pain were elicited from the TrP prior to injection, then both should be observed when the needle penetrates the TrP during injection. Hong showed that when needle penetrations of a TrP produced LTRs, those injections were much more likely to result in subsequent pain relief than penetrations that did not elicit an LTR.

Following effective needling, most TrP characteristics should have disappeared: no LTR, no evoked referred pain, and no spot tenderness should remain.

Step 6: Inject Solution

Whenever an LTR or pain response occurs, an additional 0.1–0.2 ml of procaine solution is injected to minimise postinjection soreness.

The local analgesic agent should be injected only if an LTR accompanies needle insertion. Thumb pressure on the plunger slowly introduces small amounts of 0.5% procaine solution as the needle advances within the muscle. This ensures that the procaine is present to relieve pain at the instant that the needle tip encounters an active locus of the TrP.

The needle is inserted deep enough to fully penetrate the taut band (TrP) region and then is pulled back to the subcutaneous tissue layer, but not out of the skin. A drop of 0.5% procaine (or lidocaine) is injected into the taut band following every LTR.

Step 7: Probe for Additional Loci

Sometimes a cluster of TrPs, each with a discrete taut band, are present in one part of the muscle. After each probing movement, the needle tip must be withdrawn to subcutaneous tissue and redirected before the next movement. When this probing search of the spherical region is completed, the site is palpated for any remaining spots of tenderness. If one is found, it is accurately localised with the fingers and injected. All tender spots in that region should be eliminated before withdrawing the needle through the skin.

Step 8: Haemostasis

Injecting TrPs is a full-time job for both hands of the practitioner. The injecting hand is busy placing the needle and controlling the plunger of the syringe for injection. The palpating hand constantly maintains haemostasis and often must fix the TrP to help the needle penetrate it. It also must be ready to detect any palpable LTRs.

Haemostasis is important. Local bleeding is irritating to the muscle, causes postinjection soreness, and can produce an unsightly ecchymosis which is usually preventable. As the needle is withdrawn, one finger slides over the track of the needle and instantly applies pressure where the needle was. If visible bleeding develops, pressure and a cold pack should be applied and the patient warned of a possible "bruised" spot.

To prevent bleeding, the fingers of the palpating hand should be spread apart, maintaining tension on the skin (Fig. 3.16A) to reduce the likelihood of subcutaneous bleeding where the needle has penetrated. Also, during the injection, the fingers exert pressure around the needle tip to provide haemostasis in deeper tissues.

Hong's Techniques

Hong introduced two new injection techniques: one was a safer way to hold the syringe and the other was a different way to perform the injection itself.

Holding the Syringe (Fig. 3.18): Hong recommends a way to hold the syringe that is safer than the usual way. His technique ensures that the syringe will move with the patient and not enter unintended tissue, and that the finger on the plunger of the syringe will move with the syringe and not cause an accidental injection. The syringe is held between the thumb and lesser fingers, and the plunger is depressed with the index finger. Particularly valuable when the needle is directed toward the lung or toward major arteries or nerves.

Rapid Technique ("Fast In, Fast Out"): Hong also described his "fast in, fast out" method of injecting a TrP that has been precisely located by palpation. The palpating finger should stay over or straddle the taut band in order to guide the needle insertion directly to the TrP. The needle movement is rapid, "fast in" and "fast out." Hong has modified the technique as originally described — he now takes 2 or 3 seconds between insertions. The pause following each insertion permits time to consider the tissue textures traversed by the needle and where to redirect the needle, time for identification of an LTR, and time to immediately inject anaesthetic solution into the same needle track when a twitch occurs.

This rapid technique avoids muscle fibre damage from LTRs. Experience during research studies showed that LTRs are elicited more frequently when the needle is moved quickly rather than slowly. The track of needle insertion is usually very straight and the needle is less likely to be deflected by the dense contraction knots when the needle is inserted at high speed.

Special Precautions

Pneumothorax Hazard:

NEVER aim the needle at an intercostal space — the clinician avoids the distressing complication of a pneumothorax. The only exception is when there is need to inject intercostal muscles, and this is done only with GREAT care. The patient may sneeze or jump unexpectedly.
If the lung is punctured and collapses, dyspnoea, cough, and chest pain characteristic of a pneumothorax follow.
When injecting muscles near the thorax (scalenes, serratus anterior, intercostals, paraspinals, rhomboids, etc.), always direct the needle away from the pleura.

Needle breakage hazard:

A needle is prone to break where it attaches to the hub. The needle should never be inserted solidly to its hub.
If indentation of the skin provides inadequate safe penetration, and sterilising hypodermic needles is not feasible, an alternate manual treatment approach will be required.

Needle tip location:

The location of the needle tip can readily be misjudged when using a long slender needle. It is especially important to insert the needle straight and avoid any side pressure that might bend the needle, deflecting the tip an unknown distance to one side.

How Many Injections?

Note the definition of one injection at the beginning of this section. The number of TrP sites that need to be injected per visit and the number of visits required are strongly dependent on the patient's condition and the practitioner's skill and judgement.

Recently activated (acute) myofascial TrPs that have no perpetuating factors or additional tissue damage because of mechanical injury to other tissues (i.e., TrPs that are uncomplicated) should resolve with one or two injections. This is especially true if, after injection, the patient is trained and then performs exercises to maintain full range of motion of the involved muscle(s).

When both central TrPs and attachment TrPs are present, both sites must be injected, which would count as separate injections.

When initial TrP therapy is delayed and symptoms have not subsided with time, the longer the period of delay before starting TrP therapy, the larger the number of injections that will be required over a longer period of time. Some chronic TrP problems could involve dozens of injections over months of treatment.

When there are multiple active TrPs in functionally related muscles, there is a distinct advantage to inactivating them as a group. Thus, 5 or even 10 injections at one visit can be appropriate. Since a properly performed and effective injection produces an LTR which is often associated with considerable pain, there is a limit as to how many painful injections should be performed at one visit out of respect for the patient's emotional and autonomic distress level.

Presence of unrecognised perpetuating factors will lead to unnecessary injections. Presence of associated joint dysfunctions that need manipulation can cause poor response to injection and prompt recurrence of the TrP activity.

The presence of concurrent fibromyalgia will increase the number of injections required and can justify recurrent injections every 6–8 weeks since the fibromyalgia acts as a perpetuating factor that has no cure. Inactivating their TrPs can provide significant pain relief for many of these patients.

Ligamentous Sprains

The pain of ankle and wrist sprains has been reported to be relieved in most cases by injection with procaine, either with or without epinephrine. Either 0.5% or 1% procaine is effective. Best results are obtained if all of the tender spots in the sprained joint are injected as soon as possible (less than 12 hr) following injury. The joint should be pain free following injection, which should permit use of the joint at once, including some slow walking. It should be used gently throughout its normal range to remain free of pain, aided by an elastic support to remind the patient to protect the joint.

Postinjection Procedures

Stretch following TrP injection is an integral part of that treatment. Failure to stretch following injection can mean failure of treatment.

Immediately After Injection

Immediately following injection (before the effect of injected anaesthetic has worn off) the patient should actively move each muscle injected through its FULL range of motion complete 3 times, reaching its fully shortened and its fully lengthened position during each cycle. The muscle usually feels stiff toward the end of full stretch range of motion on the first cycle, less on the second, and begins to feel comfortable through its full range by the third cycle. It is important that the patient move the muscle slowly to explore the end range of motion for additional release.

The process is facilitated if the clinician applies a few sweeps of vapocoolant spray in parallel lines over each muscle and its referred pain pattern during its first stretch. Vapocoolant application should be followed by a hot pack or pad placed over the areas sprayed with vapocoolant.

Postinjection Stretch Exercises

The postinjection stretch is important because it helps to again equalise sarcomere lengths throughout the length of affected muscle fibres, which relieves their abnormal tension and can eliminate the palpable taut bands. Voluntary movement also relieves residual stiffness at full range of motion, helps the patient to appreciate fully the improved range of motion, and provides the patient the stretches that will be incorporated in the home programme.

The practitioner should teach the patient a home exercise programme that includes the postinjection stretches which the patient just performed.

Postinjection Soreness

Lewit noted muscle soreness after dry needling and after a local anaesthetic injection, but made no mention of applying heat as part of the treatment. The postinjection soreness, per se, is not unfavourable if the patient's related pattern of referred pain has been relieved. However, it is wise to let the muscle recover completely from postinjection soreness, which ordinarily lasts at most 3 or 4 days, before injecting its TrPs again. Soreness can also be caused by ineffectually needling close to, but not into, TrPs.

For patients who are troubled by postinjection soreness, acetaminophen is usually as effective as aspirin and less irritating to the stomach.

Reasons for Failure of Injection of Trigger Points

Aside from a complete misdiagnosis, disregarding perpetuating factors is probably the most important reason for failure.
Injecting a latent TrP, not the responsible active TrP.
Injecting the area of referred pain and referred tenderness, not the TrP. This error provides only incomplete, temporary relief.
Needling the vicinity of the TrP, including needling of the taut band, but missing the TrP itself.
Using a needle finer than 25-gauge for precision injection — this may allow the tip of the needle to be shoved aside by the dense contraction knots which are at the heart of the TrP and which must be disrupted for maximum effectiveness.
Injecting a solution with an irritating or allergenic bacteriostatic preservative, such as sodium bisulphite; sodium hyposulphite is less irritating.
Inadequate haemostasis followed by irritation of the TrP due to local bleeding.
Overlooking other active TrPs that are contributing to the patient's pain.
Forgetting to have the patient perform active range of motion following injection, with the result that the muscle's full range was not incorporated into daily activities.
Omitting regular passive stretch exercises at home, which would have maintained the full length of the muscle and suppressed further TrP activity.

Blindly Needling an Area of Diffuse Tenderness

Blindly probing an area of diffuse tenderness where there is no palpable band or muscle attachment is futile. Such an area is most likely to be a pain reference zone, not a TrP. Injecting a local anaesthetic in the reference zone may temporarily reduce the referred pain, but it does not eliminate the cause of the pain.

References

This page is based entirely on: Simons DG, Travell JG, Simons LS. Myofascial Pain and Dysfunction: The Trigger Point Manual. Volume 1: Upper Half of Body. 2nd ed. Baltimore: Williams & Wilkins; 1999. Chapter 3: Apropos of All Muscles, Section 13, pp. 150–166.

Concept:Trigger Point Injection