Trigger points are the result of local contraction of a small number of muscle fibers within a muscle, as opposed to contraction of the entire muscle. The taut bands in a trigger point are due to the shortened sarcomeres in a muscle fiber. Numerous factors can contribute to the development of trigger points. Among these factors are muscle strain, postural or mechanical imbalance, overuse syndromes, muscle overload, direct trauma, and physical or psychological stress. Muscle damage is not a requisite for the development of trigger points, but biochemically, there may be associated injury to cell membranes or sarcoplasmic reticulum resulting in release of calcium-ions, and disruption of cytoskeletal proteins [1]. Presence of excess calcium-ions interferes with release of actin from myosin and hence with relaxation of muscle fibers. Sustained contraction may cause localized muscle ischemia, which in turn may result in lower pH and release of inflammatory mediators in muscle tissue. Another theory suggests that there is release of excessive acetylcholine, which can result in increased muscle fiber contractions [1].

Trigger point injections are often used to treat myofascial pain. Many studies have been performed to evaluate the outcomes of trigger point injections. One study showed that lidocaine injection into myofascial trigger points improves pain and quality of life [2]. Another study found that injection of lidocaine was superior to placebo or dry needling in the short term [3]. Trigger point injections can also lead to statistically significant improvements in pain, range of motion, and depression scores with both local anesthetic injection and dry needling of trigger points [4]. A prospective randomized evaluation of treatments for myofascial trigger points argues that dry needling provides as much relief as when local anesthetic or corticosteroid are injected [5]. Therefore, local anesthetic injection, steroid injection, and dry needling have all be shown to be effective options to treat trigger points.

Trigger points are identified by palpation over the painful region. A taut band of muscle is identified. Palpation over this region produces both local and radiating pain that is concordant to the patient’s subjective report of his own pain symptoms. Though some may use EMG or nerve stimulation to confirm intramuscular needle placement and ultrasound to visualize affected muscles and taut bands, the procedure is most commonly performed without the need for additional equipment [6]. Placement of the needle into the trigger point may produce a visible and palpable muscle “twitch response.” Once the needle is in place, medication can be injected or dry needling can be performed. Some advocate directing the needle in multiple planes within a trigger point area in order to maximize the mechanical breakdown of the taut band. Some advocate the use of other injectates in trigger point injections such as botulinum toxin or prolotherapy. Platelet-Rich Plasma (PRP) is another option for the injection of painful soft tissue, but injection over ligamentous, tendinous and articular injuries are preferable to injection over muscle.

Botulinum toxin inhibits the ability of muscle to contract by interfering with presynaptic release of acetylcholine. Studies have not demonstrated superiority of botulinum toxin over other forms of injectate [3, 7]. One study suggested that the degree and duration of pain relief, improved function, and patient satisfaction was comparable between injection of Botox and injection of local anesthetic. However, botulinum toxin injection seems to improve pain scores and quality of life when compared to dry needling [2]. In a review of five clinical trials, one trial concluded that botulinum toxin was effective in reducing pain, while four concluded that it was not [8]. Another study injected botulinum toxin directly into trigger points but did not improve cervicothoracic myofascial pain [9]. It is also important to note the risk of dysphagia and other adverse effects that are associated with the use of the botulinum toxin. While cost of botulinum toxin can be a drawback and results of various studies are mixed, botulinum toxin injection into trigger points could be considered after patients have tried other measures without efficacy. Botulinum toxin is FDA approved for the treatment of chronic migraines, upper limb spasticity, cervical dystonia and detrusor muscle herperactivity. Any other intramuscular utilization of Botulinum toxin is considered an “Off-Label” use [10, 11].

Prolotherapy is a treatment for musculoskeletal pain that involves the injection of a substance, commonly dextrose, phenol, and or sodium morrhuate solution, to initiate an inflammatory reaction in soft tissue. The theory behind this treatment involves the release of inflammatory mediators that eventually recruit fibroblasts and other local growth factors to the injected areas. The influx of growth factors in theory helps rebuild stronger, healthier soft tissue, though this is yet to be fully established [12]. Patients should expect potential increased pain, swelling, and the redness for several days following the injection. These reactions signify the inflammatory and proliferative processes that the injection was intended to trigger. Risk of the treatment includes infection and increased pain with greater than intended inflammation. However, the risk is generally low, and prolotherapy can be considered for treatment of painful musculoskeletal conditions when other treatments have failed.

The PRP injection is a form of prolotherapy in that it also involves injecting a substance to promote growth factors [13]. The injectate making up PRP is an autologous blood product obtained through a centrifugal process that isolates a high concentration of platelets and growth factors from whole blood [14]. Many manufacturers market devices that utilize the centrifugal process to isolate PRP products, which are in turn injected into soft tissues or joints. This process allows clinicians to inject supra-physiologic concentrations of growth factors to stimulate and accelerate tissue proliferation. Its use in musculoskeletal medicine has increased as evidence emerges that it may augment healing in muscles, tendons, and ligaments [15].

While overall evidence for prolotherapy and PRP are conflicting, the most promising evidence for prolotherapy and PRP has been with the treatment of chronic low back pain and ligamentous injuries and tendinopathy. A Cochrane Database review concluded that prolotherapy may help improve chronic low-back pain and disability when used adjunctively with other treatments. Prolotherapy can also effectively treat refractory tendinopathies, such as in studies on lateral epicondylosis and Achilles tendinopathy. A single-blinded randomized clinical trial concluded more rapid symptom improvement in patients receiving prolotherapy and eccentric exercises for Achilles tendinopathy than with eccentric exercises alone [16]. A systematic review of injection therapy including prolotherapy and PRP for lateral epicondylitis concluded that there is strong pilot-level evidence supporting the use of prolotherapy and PRP [17]. A retrospective study evaluating PRP at various sites of chronic tendinopathy noted an overall 75 % decrease in pain, and 85 % patient satisfaction rate [18]. There is emerging evidence supporting prolotherapy and PRP in cases of osteoarthritis [17, 19]. A randomized-controlled trial showed that PRP was better than placebo in treating symptoms in patients with early knee osteoarthritis, but also noted that the benefits began to deteriorate after 6 months [20]. Preliminary results of another randomized-controlled trial suggest that PRP improves clinical symptoms of moderate osteoarthritis at 1 year follow-up. The same study did not find statistically significant differences between the benefits of PRP and hyaluronic acid, though there was a trend towards PRP in patients with lower grade osteoarthritis [21]. A separate prospective comparative study concluded that PRP showed higher and longer-lasting improvements in pain, symptoms, and recovery of articular function [22]. There are also some studies on small animals using PRP to treat intervertebral disc degeneration, with a trend towards a protective effect on damaged discs especially when utilized earlier in the disease process [13–27]. However, existing studies are limited by small sample size and design. In addition, high-level clinical trials that prove the efficacy of PRP remains lacking, especially when comparing PRP to other forms of injections and when considering the most appropriate volume, preparation, timing, and number of injections. Further research on the topic will help develop protocols and indications.

Facet joint injections, medial branch blocks, and medial branch radiofrequency nerve ablation are strategies employed to treat pain from the zygapophysial (facet) joints. Facet joints are true synovial joints composed of the superior and inferior articular processes of adjacent vertebrae surrounded by a fibrous capsule.

The epidural space is the area surrounding the dural sac in the spinal canal and extending to the brain. In the spinal canal, the epidural space is bordered anteriorly by the posterior longitudinal ligament, laterally by the pedicles, neuroforamina and nerve roots, and posteriorly by the lamina and ligamentum flavum. Epidural fat and blood vessels are present in certain areas within the epidural space. Spinal nerve roots exit the vertebral column through the vertebral neuroforamen, making this an area of interest for targeted injections. The cervical neuroforamen faces obliquely anterior and lateral, whereas the thoracic and lumbar neuroforamen faces obliquely posterior and lateral. Neuroforamen are bordered superiorly and inferiorly by the pedicles of the adjacent vertebra, anteriorly by vertebral bodies and intervertebral discs, and posteriorly by the facet joints.

The SI joint is formed by the sacrum and the ilium. The majority of this apposition is formed by a fibrocartilaginous connection. However, inferoposteriorly, there is a small true synovial joint. SI injections are performed targeting this space.