Trismus, commonly known as "lockjaw," is a condition of restricted mouth opening, often caused by a tonic spasm of the muscles of mastication. While it can result from various factors, an iatrogenic (medically-caused) form is a recognized complication in dentistry. The inferior alveolar nerve block (IANB), the most frequently performed injection in dentistry for anesthetizing the lower teeth, is the local anesthetic procedure most commonly linked to this condition.
The Inferior Alveolar Nerve Block and Trismus
During an inferior alveolar nerve block, the anesthetic is injected into the pterygomandibular space, an area that is bordered by the medial pterygoid muscle. Inaccurate placement or deep injection can lead to complications such as nerve damage or muscle irritation. The medial pterygoid muscle, a strong jaw-closing muscle, is particularly susceptible to trauma during this procedure.
The most common cause of post-injection trismus is a medial pterygoid myospasm, an involuntary spasm of this muscle that occurs secondary to the nerve block. The pain triggered by stretching the affected muscle causes a reflex contraction, further limiting the patient's ability to open their mouth.
Mechanisms of Trismus Following Injection
Several factors can contribute to the development of trismus after an inferior alveolar nerve block:
- Intramuscular Injection: Accidental injection directly into the muscle tissue can cause localized irritation and inflammation. This trauma can lead to muscle swelling and a protective spasm.
- Hematoma Formation: A needle may injure blood vessels, such as the inferior alveolar artery or vein, leading to the formation of a hematoma within the pterygomandibular space. The subsequent swelling and pressure can restrict muscle movement and cause pain.
- Needle-Track Infection: In rare cases, a low-grade infection may be introduced along the needle path. The resulting inflammation and pus formation can cause persistent trismus that may require antibiotics or surgical drainage.
- Local Anesthetic Myotoxicity: The anesthetic agent itself can have a direct toxic effect on muscle tissue, a phenomenon known as myotoxicity. While typically temporary, it can cause muscle degeneration and inflammation, contributing to trismus.
- Repeated Injections: Administering multiple injections in a short period in the same anatomical area increases the risk of tissue trauma, bleeding, and fibrosis.
Risk Factors and Prevention
Multiple factors can increase a patient's risk of developing post-injection trismus. Awareness of these risks is crucial for both the clinician and the patient. In addition to the injection technique-related factors, a patient's medical history can also play a role.
To prevent trismus, dental professionals should follow proper injection techniques and have a thorough understanding of the local anatomy. Avoiding multiple injections in the same site, especially in a compromised area, and ensuring a sterile field are also important preventive measures. Patients can minimize their risk by maintaining good oral hygiene and informing their dentist of any pre-existing jaw or muscle conditions.
Comparison of Nerve Blocks and Trismus Risk
While the IANB is the most common cause of post-injection trismus, other blocks carry different levels of risk or are associated with trismus in specific contexts.
| Feature | Inferior Alveolar Nerve Block (IANB) | Maxillary (Posterior Superior Alveolar) Blocks | Local Infiltration (Maxillary) |
|---|---|---|---|
| Trismus Risk | Most common due to deep muscle penetration | Very rare; involves soft tissue rather than large masticatory muscles | Not a significant risk as it involves superficial tissue and no deep muscle penetration |
| Causative Factor | Accidental muscle trauma, hematoma, or infection in the pterygomandibular space | Primarily associated with other complications, like hematoma in the infratemporal fossa, not typically trismus | Generally a non-issue unless infection spreads or in rare cases of myotoxicity |
| Associated Muscle | Medial Pterygoid muscle is the primary muscle involved in spasm | Not directly associated with the masticatory muscles | Does not involve the deep masticatory muscles |
| Preventive Measures | Proper technique, anatomical knowledge, avoiding multiple injections, gentle insertion | Correct technique to avoid vascular injury | Standard sterile technique |
Management and Treatment of Trismus
Treatment for post-injection trismus is generally conservative and focuses on pain relief and restoring function. Early and consistent intervention is key to preventing the condition from becoming chronic.
Initial management often includes:
- Moist Heat Application: Applying a warm, moist compress to the affected side of the jaw for 15–20 minutes every hour can increase blood flow and relax muscles.
- Soft Diet: A diet of soft foods minimizes the stress on the masticatory muscles, aiding in their recovery.
- Analgesics: Over-the-counter nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen, can reduce pain and inflammation.
For more persistent cases, or as part of a comprehensive plan, a healthcare provider may recommend:
- Muscle Relaxants: Prescription muscle relaxants may be prescribed to help reduce muscle spasms.
- Jaw Exercises: Gentle, regular stretching of the jaw muscles can improve range of motion. Exercises include passive stretching with fingers or active opening and closing.
- Jaw Mobilizing Devices: Specialized devices, like the TheraBite, can aid in passive stretching and help patients regain full mobility.
- Physical Therapy: A physical therapist can guide patients through specific exercises and stretches to improve jaw function and reduce pain.
- Antibiotics: If an infection is suspected, antibiotics will be prescribed.
Conclusion
While dental procedures are generally safe, understanding potential complications like trismus is essential for effective patient care. The inferior alveolar nerve block is the most common nerve block that causes trismus, primarily due to potential trauma to the medial pterygoid muscle. By adhering to proper injection techniques and implementing early, conservative management strategies involving heat therapy, medication, and jaw exercises, most cases of post-injection trismus resolve within a few weeks. However, persistent cases may require more extensive treatment. For further insights into dental anesthesia complications, a resource from the National Institutes of Health (NIH) provides in-depth research.
