Introduction to Midcarpal Arthroscopy
Wrist arthroscopy has evolved from a purely diagnostic tool into an indispensable therapeutic modality in modern orthopedic surgery. While radiocarpal arthroscopy provides excellent visualization of the proximal carpal row and the triangular fibrocartilage complex (TFCC), the midcarpal examination is the definitive gold standard for assessing carpal instability, intrinsic ligament integrity, and midcarpal chondral lesions.
The midcarpal joint, a complex articulation between the proximal and distal carpal rows, dictates a significant portion of global wrist kinematics. Pathologies within this space—most notably scapholunate (SL) and lunotriquetral (LT) interosseous ligament tears—often present with subtle radiographic findings but profound clinical morbidity. A meticulous, systematic arthroscopic evaluation of the midcarpal joint allows the orthopedic surgeon to dynamically assess ligamentous competency, grade instability using the Geissler classification, and formulate an evidence-based operative plan.
Clinical Pearl: Radiocarpal evaluation alone is insufficient for diagnosing intrinsic ligament tears. A tear may appear partial from the radiocarpal joint but demonstrate complete, dynamic instability when viewed and probed from the midcarpal portals. Always perform a midcarpal examination when carpal instability is suspected.
Regional Anatomy and Biomechanics
A profound understanding of midcarpal anatomy is prerequisite for safe portal placement and accurate diagnostic interpretation. The midcarpal joint is a continuous, S-shaped articular space.
The Proximal and Distal Carpal Rows
- Proximal Row: Functions as an intercalated segment between the radius and the distal carpal row. It consists of the scaphoid, lunate, and triquetrum. Its motion is entirely dependent on the mechanical forces exerted by the surrounding articulations and ligaments.
- Distal Row: Consists of the trapezium, trapezoid, capitate, and hamate. These bones are tightly bound by robust interosseous ligaments, functioning essentially as a single biomechanical unit that moves synchronously with the metacarpals.
Intrinsic Ligaments
The stability of the proximal row relies heavily on two critical intrinsic ligaments:
1. Scapholunate Interosseous Ligament (SLIL): Composed of dorsal, proximal (membranous), and volar regions. The dorsal band is the thickest and most biomechanically significant for preventing dorsal intercalated segment instability (DISI).
2. Lunotriquetral Interosseous Ligament (LTIL): Composed of similar regions, but the volar band is the most robust, preventing volar intercalated segment instability (VISI).
Preoperative Planning and Patient Positioning
Operating Room Setup
The procedure is typically performed under regional anesthesia (supraclavicular or axillary block) or general anesthesia, depending on patient preference and anticipated surgical duration.
* Positioning: The patient is placed supine with the operative arm extended on a radiolucent hand table.
* Tourniquet: A well-padded pneumatic tourniquet is applied to the proximal arm and inflated to 250 mm Hg after exsanguination.
* Traction: The hand is placed in a dedicated wrist traction tower. Sterile finger traps are applied to the index and long fingers (and occasionally the ring finger for ulnar-sided pathology). Approximately 10 to 15 pounds of longitudinal traction is applied to distract the radiocarpal and midcarpal joints.
Surgical Warning: Excessive traction (greater than 15 pounds) or prolonged traction (exceeding 2 hours) significantly increases the risk of iatrogenic neurapraxia, particularly to the digital nerves or the superficial branch of the radial nerve.
Portal Topography and Establishment
Precise portal placement is the cornerstone of successful wrist arthroscopy. The midcarpal portals are established after the radiocarpal examination is complete.
The Radial Midcarpal (MCR) Portal
The MCR portal is the primary viewing portal for the midcarpal joint.
* Location: The portal for entry into the radial side of the midcarpal joint is located approximately 1 cm distal to the standard 3-4 radiocarpal portal. It is situated to the radial side of the third metacarpal axis and proximal to the palpable soft depression between the scaphoid and the capitate.
* Establishment Technique:
1. Identify the anatomic landmarks via palpation under traction.
2. Insert an 18-gauge needle into this portal with a slight volar tilt (approximately 10 to 15 degrees) to match the anatomic slope of the midcarpal joint.
3. Distend the joint with 5 to 7 mL of sterile normal saline. A distinct "pop" or loss of resistance is felt as the needle breaches the capsule.
4. Incise the skin over this area using a #11 blade, taking care to cut only the dermis to avoid injury to the terminal branches of the posterior interosseous nerve or superficial radial nerve.
5. Dissect bluntly down to the capsule using a small hemostat to spread the subcutaneous tissues.
6. Insert a blunt trocar and cannula, permitting fluid inflow through the arthroscope.
The Ulnar Midcarpal (MCU) Portal
The MCU portal serves as the primary working portal for instrumentation (probing, debridement, or thermal shrinkage) during midcarpal evaluation.
* Location: The ulnar midcarpal portal is located in the center of the axis of the fourth metacarpal, just proximal to the capitohamate joint.
* Establishment Technique:
1. With the arthroscope remaining in the MCR portal (viewing the ulnar aspect of the midcarpal joint), enter the MCU location with an 18-gauge needle.
2. Verify the position of the needle by direct intra-articular vision. The needle should enter to the radial side of the extensor digitorum communis tendons.
3. Once the trajectory is confirmed visually, make a superficial skin incision, perform blunt dissection, and introduce the working cannula or instruments.
Pitfall: Blind insertion of the MCU portal without direct visualization from the MCR portal can lead to iatrogenic scuffing of the capitohamate articular cartilage or damage to the delicate interosseous ligaments.
Systematic Midcarpal Diagnostic Sweep
Once the MCR portal is established and the arthroscope is introduced, a systematic, reproducible diagnostic sweep must be performed to ensure no pathology is overlooked.
1. The Scaphocapitate and STT Articulations
- Place the arthroscope through the MCR skin incision to view the convex head of the capitate distally and the concave distal pole of the scaphoid proximally.
- Evaluate the articular cartilage for chondromalacia, fibrillation, or exposed subchondral bone.
- Scaphotrapezialtrapezoid (STT) Joint: By moving the arthroscope toward the radial side along the scaphocapitate joint, the STT joint can be examined. This is a common site for early osteoarthritis. Assess the congruency and cartilage health of the distal scaphoid articulating with the trapezium and trapezoid.
2. The Scapholunate (SL) Interval
- Retract the arthroscope slightly and direct the lens ulnarly to visualize the SL interval.
- This is the critical step for evaluating SLIL integrity.
- Introduce a probe via the MCU portal. Palpate the SL articulation.
- Geissler Classification of Instability:
- Grade I: Attenuation or hemorrhage of the interosseous ligament; no step-off; probe cannot be passed through the interval.
- Grade II: Incongruency or slight step-off of the carpal interval; probe can be passed partially into the space.
- Grade III: Step-off is present; probe can be passed completely through the interval between the scaphoid and lunate (the "drive-through" sign).
- Grade IV: Massive instability; the arthroscope itself can be passed through the SL interval from the midcarpal to the radiocarpal joint.
3. The Lunotriquetral (LT) Interval
- Moving further in an ulnar direction along the proximal carpal row, examine the lunotriquetral joint.
- Use the probe from the MCU portal to apply volar and dorsal pressure on the triquetrum while observing the lunate.
- Assess for step-off or abnormal translation, applying the same Geissler grading system used for the SL interval.
4. The Capitohamate Articulation
- Direct the arthroscope distally to evaluate the articulation between the capitate and the hamate.
- Traction and dynamic manipulation of the wrist (flexion, extension, radioulnar deviation) by an assistant allow for better inspection of these joints, revealing occult dynamic instabilities.
Surgical Technique 69-3: Comprehensive Radiocarpal and TFCC Assessment
While the midcarpal examination evaluates the intrinsic ligaments, a complete wrist arthroscopy mandates a rigorous evaluation of the radiocarpal joint, specifically focusing on the Triangular Fibrocartilage Complex (TFCC) and the extrinsic palmar ligaments.
Evaluating the TFCC
The TFCC is the primary stabilizer of the distal radioulnar joint (DRUJ) and the ulnar carpus.
* With the arthroscope in the 3-4 portal and a probe in the 4-5 or 6R portal, palpate the TFCC to determine its structural integrity.
* The Trampoline Test: Press the probe against the articular disk of the TFCC. A normal, intact TFCC will bounce back like a trampoline. A torn or detached TFCC will feel soft, compliant, and lack this elastic rebound.
* The Hook Test: Attempt to pull the ulnar attachment of the TFCC radially. If the TFCC pulls away from the fovea or ulnar styloid, a peripheral tear (Palmer Class 1B) is present. Pay special attention to its attachment to the ulnar margin of the radius (Palmer Class 1D tears).
Ulnocarpal and Palmar Ligament Assessment
- Moving toward the ulnar side of the wrist, identify the ulnocarpal ligaments (ulnolunate and ulnotriquetral ligaments) and the proximal articular surface of the triquetrum. Assess for tears or attenuation, which may indicate ulnar impaction syndrome.
- Insert a probe in portal 4 or 5 to evaluate the stout palmar carpal ligaments (e.g., radioscaphocapitate, long radiolunate, short radiolunate).
- Evaluate the proximal surfaces of the scapholunate and lunotriquetral interosseous ligaments from this radiocarpal perspective.
- Note: It may be necessary to move the probe into a more radial portal (such as the 1-2 or 3-4 portal, switching the arthroscope to the 4-5 portal) to adequately examine the radial-sided structures or the radial insertion of the TFCC.
Postoperative Protocol and Closure
Meticulous closure and postoperative care are vital to prevent complications and ensure optimal recovery.
Joint Clearance and Hemostasis
- After the diagnostic examination and any concurrent operative procedures (e.g., debridement, thermal shrinkage, or TFCC repair) have been completed, perform a final sweep of both the radiocarpal and midcarpal joints.
- Determine that no loose objects, cartilage fragments, or broken instruments are left within the joint.
- Remove the arthroscope, instruments, and all drainage tubing.
- Deflate and remove the tourniquet. Obtain meticulous hemostasis. While arthroscopic portals rarely bleed significantly, small subcutaneous vessels can cause postoperative hematomas if ignored.
Portal Closure and Dressing
- Close the portal incisions using non-absorbable skin sutures (e.g., 4-0 nylon or Prolene) or surgical staples, depending on surgeon preference. Avoid deep dermal sutures that might tether the extensor tendons.
- Analgesia: Infiltration of the joint and the portal sites with a long-acting local anesthetic agent (such as 0.5% bupivacaine without epinephrine) significantly helps minimize postoperative pain and reduces the need for systemic opioids.
- Immobilization: Apply a sterile, non-adherent dressing over the portals, followed by a bulky hand dressing. Immobilize the wrist with a volar short-arm splint in neutral position.
Rehabilitation
- Finger motion is encouraged immediately in the recovery room to prevent tendon adhesions and reduce edema.
- The splint and sutures are typically removed at 7 to 14 days postoperatively.
- Subsequent rehabilitation protocols are dictated by the specific operative procedures performed (e.g., immediate active range of motion for simple debridement vs. 4-6 weeks of cast immobilization for a TFCC repair). Arthroscopic procedures for specific conditions are described in the sections discussing those respective pathologies.
Complications and Pitfalls
While wrist arthroscopy is generally safe, the surgeon must be vigilant regarding potential complications:
* Nerve Injury: The superficial branch of the radial nerve (SBRN) and the dorsal branch of the ulnar nerve (DBUN) are at highest risk during portal placement. Strict adherence to blunt dissection techniques is mandatory.
* Tendon Injury: Extensor tendons can be lacerated by rogue scalpel blades or abraded by aggressive use of motorized shavers.
* Fluid Extravasation: Excessive fluid pressure can lead to severe forearm edema and, in rare cases, compartment syndrome. Always monitor inflow pressure and ensure adequate outflow.
* Cartilage Scuffing: Iatrogenic damage to the articular cartilage occurs when portals are established blindly or when instruments are forced into tight joint spaces without adequate traction.
