Clinical and Functional Anatomy of the Scaphoid and Lunate

Figure 1 and 2 show the posting of scaphoid and lunate ones in relation to associated wrist structures. 

The scaphoid and lunate are two eight carpal bones in the hand. The distal surface of the radius and proximal row of carpal bones are involved in the articulation of the radoiocarpal joint, a synovial ellipsoid joint, allowing movements of flexion-extension and adduction-abduction; majority of abduction arises from the mid-carpal joint. Motion between both the radiolunate and radioscpaahoid joints are vital in efficent flexion of the wrist. Positioned in the mobile proximal row of carpal bones along with the triquetrum and pisiform bones. The scaphoid articulate with the trapezium and trapezoid of the distal carpal bones. The scapholunate ligament runs between the two bones, as well as providing a point of attachment of the fibrous joint capsule that surrounds the wrist joint. Throughout this course, identification of the scaphoid bone is done by the bone shaded orange, and the carpal bone shaded green is the lunate.

The Radoiocarpal Joint

The Radiocarpal Joint

  • The radoiocarpal joint is a synovial ellipsoid joint, that involves the articulation between the distal surface of the radius and articular disc and the proximal surfaces of the proximal row of carpal bones; the scaphoid, lunate and triquetral
  • It is covered in a fibrous capsule, lined by a synovial membrane and is strengthened by palmar, dorsal ligaments and collateral ligaments. 
  • The joint is supplied by branches of the anterior interosseous artery, branches of the radial and ulnar arteries and dorsal metacarpal arteries. 

Movements of the Radoiocarpal Joint

Neutral

Figure 3. Shows the position of the radius and articulating carpal bones, in a neutral position. 

Extension

Also known as dorsiflexion. Figure 4 shows the position of the scaphoid bones in relation to the radius and capitate during extension of the wrist. This movement occurs about a transverse axis in the sagittal plane. Maximum range of extension is 85°. This is due to the posterior margin of the tibial extending more distally than the anterior margin of the tibia, which limits during flexion. During extension, twisting of the scaphoid that occurs, results in the more prominent nature of the tubercle on full extension. It should be noted that the movement of the proximal row of carpal bones glide in the opposite direction to the hand movement 

Flexion

Also known as palmar flexion. Figure 5 shows the position of the scaphoid and lunate in relation to the radius and the captivate; an associated carpal bone during flexion of the wrist. The maximum range of flexion is 85° and is limited by the anterior region of the tibia. During flexion the scaphoid and lunate articulate with the distal surface of the radius. Also during flexion, the scaphoid twists about its long axis, resulting in the tubercle to be less prominent during full flexion. 

Abduction

Figure 6 indicates the movement of abudction of the radoiocarpal joint. This type of movement is also known as a radial and ulnar deviation and occurs in the frontal plan. Abduction is the lateral movement of the carpal bones in relation to the distal end of the radius. Abduction is limited by the radial styloid process which extends further than the ulnar styloid process, medially. Therefore only 15° of abduction can be achieved.



Adduction

Figure 7 indicates adduction of the joint. In a similar way as abduction, adduction is also known as radial and ulnar devotion, occurring in the frontal plane. Adduction is the medial movement of the carpal bones in relation to the distal end of the radius. Adduction is limited by the ulnar styloid process, which does not extend distally as far as the the radial styloid process, allowing for 40-45° of adduction to occur. In adduction the scaphoid rotates, in way which moves the tubercle away from the radial styloid process, allowing the lunate the move laterally so that it lies distal to the radius. 

The Carpal Bones

Shown in figure 8, is the anatomical arrangement of the carpal bones of the right wrist into the proximal and distal row. The scaphoid is seen to lie opposite the lateral aspect of the radius and the lunate opposite the medial aspect. Synovial plane joints exist between  the distal aspects of the scaphoid, lunate and triquetral bones. However, due to the presence of interosseous and intercarpal ligaments there is minimal movement. 

Midcarpal Joint

  • Movements at the wrist not only occur at the radoiocarpal joint itself, but also at the mid carpal joint.
  •  The midcarpal joint is a complex synovial joint, which involves the articulation between the distal surfaces of the scaphoid, lunate triquetral with the proximal surface of the distal row of carpal bones; trapezium, trapezoid, capitate and hamate.
  • The carpal bones are connected by a number of ligaments, one of which being the flexor retinaculum. 
  •  This joint enclosed by a fibrous capsule, reinforced by dorsal and palmer intercarpal and ulnar and radial collateral ligaments. 
  • The carpal joints are supplied by posterior carpal branches of the radial and ulnar arteries, as well as the anterior interosseous artery. 

General Anatomy of the Scaphoid and Lunate

Anatomical Characteristics

Figure 9 shows a palmar view of the scaphoid and lunate bones, in relation to the remainder of the carpal bones

Scaphoid

Figure 10, depicts anatomical characteristics of the scaphoid bone

  • The scaphoid bone is positioned on the lateral aspect, the thumb side of the wrist. 
  • It is important in both stability and motion of the wrist joint.
  • Scaphoid comes from the greek term for boat, as the shape of the bone resembles.
  •  The scaphoid is the largest bone in the proximal row of carpal bones. 
  • The anteriorly directed tubercle on the lateral palmar surface of the scaphoid forms part of the lateral side of the carpal arch.
  • It's long axis is positioned distally and radially in direction. There is an anterolaterally directed tubercle on the lateral, distal aspect of the palmar surface, which provides attachment sites for the flexor retinaculum and abductor policies braves muscle and is crossed by the tendon of flexor carpi radialis. The region which articulates with the radius is convex and proximal. 

Lunate

  • Figure 11 shows the lunate bone.
  • The lunate bone is half-moon/cresent shaped. 
  • It articulates with the medial aspect of the distal tibia proximally and with the capitate bone of the distal row of carpal distally.   

Anatomical Relations

Figure 12 shows the arrangement of proximal row of carpal bones. From, left to right: Scaphoid, lunate, triquetral and positioned on top of this, is a sesamoid bond called the pisiform bone. 

Figure 13. Shows a dorsal view of the right hand showing the relative position of the scaphoid and lunate in the hand and the position of the anatomical snuffbox. The anatomical snuffbox is a depression in the skin near the base of the thumb. This is an important characteristic in this course and the reasons why will be highlighted later. 

Figure 14. Shows a palmar view of the right hand showing the relative position of the scaphoid and lunate in the hand and position of anatomical snuffbox 

Figure 15. Shows a dorsal view of the left scaphoid bone. The red arrows indicate the sites of articulation the carpal bones associated with the scaphoid. 

Figure 16. Shows a palmar view of the left scaphoid bone. Similarly with the red arrows indicating points of articulation of neighbouring carpal bones. 

Figure 17. Shows the distal medial region of the left lunate, indicating articular surfaces of associated carpal bones.

Figure 18. Shows the proximal lateral aspect of the left lunate. Indicating the points at which the radius and scaphoid associate with the bone. 

Blood Supply

Blood Supply to the Scaphoid

Figure 19. Represents both a dorsal and solar view, of the route of the radial artery, prior to entry to the scaphoid bone as the distal aspect of the bone. 

  • Blood supply to the scaphoid is very poor due to it being 80% covered by cartilage, which does not allow for a large number of vessels to enter the bone
  • The scaphoid bone has very poor vascularity, therefore, in the event of bone fracture, the healing process is long, taking longer to heal the bigger limb bones in the body. In majority of individuals, blood supply to the scaphoid is from distal to proximal regions. Because of the manner in which vessels enter the bone, it may result in a fracture fragment being deprived of blood supply.
  • Blood supply to the scaphoid is primarily from branches of the radial artery; the dorsal and solar branches. The solar branch enters the bone at the distal pole and vascularises 20-30% of the bone at the distal region. Th dorsal branch supplies the the remaining 70-80% of the bone and enters near the wrist. All bloody supply to the proximal third of the scaphoid is obtained from the distally positioned dorsal branch. 
  • Due to the distribution of blood to the bone, a fracture though the waist of the scaphoid may interrupt the dorsal blood supply, producing an avascular proximal pole, as it is no longer able to receive blood from the dorsal branch, as it enters at the distal pole of the bone. 
  • Most of the arterial foramina in the scaphoid bone are located around the waist area of the bone. 

Wrist Ligaments

Wrist Ligaments

Figure 20. Shows  the palmar, extrinsic and intrinsic  ligaments of the right hand, vital in the stability of carpal bones in the hand.  

Figure 21. Shows is the dorsal, extrinsic and intrinsic of the right hand

Radial Deviation

Figure 21. Shows the changes in alignment of the ligaments during radial deviation, which involves medial movement.  

Ulnar Deviation

Figure 22. Shows the changes in alignment of the ligaments during ulnar deviation, which involves lateral movement.

Radioscaphocapitiate

This ligament originates from the radial styloid process and palmar aspect of the radius and inserts onto the lateral aspect of the waist of the scaphoid, as well as onto the distal pole of the scaphoid. 

Radiolunate

Long head the radiolunate ligament indicated by left, originating next to the radioscapholunate ligament and inserting onto the palmar region of the lunate

Radioscapholunate

The radioscapholunate ligament contains neuromuscular structures and is covered by a synovial lining .

Ulnolunate

The ulnolunate ligament originates from the palmar aspect of the ulna and inserts onto the palmar horn of the lunate.

Clinical Anatomy of the Scaphoid

Scaphoid Fracture

  • Figure 23. Represents a downwards fall, onto a dorsiflexed wrist, resulting in fracture of the scaphoid bone.
  • Scaphoid Fractures are one of the most commonly fractured bones of the wrist, due to its size and shape and position within the two rows of carpal bones. The fracture out likely to occur transversely across the anatomical waist of the bone. Pain and swelling usually occurs on the lateral side of the wrist at the region of the anatomical snuffbox, particularly during dorsiflexion and abduction of the hand. Scaphoid fractures are hard to diagnose, and fractures often go mis or undiagnosed and it is not until 2 weeks after injury that a radiograph will show the fracture. 
  • 70% of scaphoid fractures occur through the waist, 20% through the proximal third of the bone and the remaining 10% occur through the distal third. 

Cause of Fracture

A fall onto an outstretched hand can result in force being transmitted through the distal row of carpal bones to the scaphoid , resulting in a fracture of the scaphoid across its anatomical waist. Severity of the fracture is classified according to the displacement go the fragments of bones have moved out of their normal position. 

Diagnosis + Treatment

  • Scaphoid fractures are often challenging to diagnosis, as the fracture line is not always revealed on an X-ray, as the scaphoid bone is sometimes hidden by asscoiated carpal bones on an X-ray. In addition to this, scaphoid fractures often do not show up on X-ray until 10-14 days after the event of injury and it is then that the healing process will have begun and the fracture site can be seen. 
  • MRI scan is suggested if X-ray still remains unclear
  • Immobilisation of the joint by use of a cast for 6-12 weeks is the most common treatment method. Surgical methods are also considered, depending on the severity of the fracture.
  • Prompt and correct diagnosis of scaphoid injury can reduce complications 

Non-Displace Fracture

Bone fragments are separated but are still lined up correctly.

Displaced Fracture 

In a displaced fracture the bone fragments have moved out of their normal position. Gaps between the fragments of bone or overlapping of fragments may result. Surgical screws, specifically used in the wrist joint are called Herbert Screws. These are often used in the event of a displaced fracture, to hold the bone fragments in the correct position for healing 

Delayed-Union

Due to the poor bloody supply to the proximal region of the scaphoid, union of the fragmented aspects takes at least 3 months to heal. This is an exceptionally long time period for such a small bone in the body. Even with immobilisation by cast, in some cases it takes a considerable amount of tie for the fracture to fully repair.

  • Figure24. Shows a fracture through the was it of the scaphoid, prevents the passage of blood from the distal to the proximal pole. This results in the proximal pole being deprived of blood; oxygen, nutrients. 
  • As a result avascular necrosis occurs at t he proximal pole of the scaphoid, due to the inability of the blood supply to reach the proximal pole, as the distally entering blood supply halted by the fracture point. 

Malunion

This is when the bone fragments heal in the incorrect position. If malunion occurs, it can result in difficult when moving the wrist, as well as pain when trying to grip or hold objects. Surgery, involving re-breaking of the bone is the primary treatment method in order to repair the event of malunion of the fracture, into the correct position. Similarly to other fracture complications, if malunion is not effectively treated it can lead to the development of osteoarthritis.

Avascular Necrosis

Due to the the direction in which blood supplies penetrates the scaphoid, it can result in a fracture fragment being deprived of blood supply, which in turn can lead to avascular necrosis. This is pathological death of bone that results to due inadequate blood supply, leading to early onset of the degenerative joint disease; osteoarthritis.

Non-Union

This is a result of failure of a scaphoid fracture to heal. The bony fragments are still completely separated. Blood supply is important during fracture healing, since blood carries the oxygen and nutrients required for effective healing. Therefore, when the blood supply is disrupted by the fracture, failure to repair may result. The early onset of osteoarthritis may occur as a result of non-union. 

Clinical Anatomy of the Lunate

Anterior Dislocation of the Lunate

  • Figure 24. Shows the position of a normal alignment of carpal bones in relation to the radius (left) and shows a dislocated lunate bone, as it sits displaced from its associated bones (right). 
  • The lunate bone is the most likely carpal bone to be dislocated, however it is a relatively uncommon event to suffer. 
  • It normally occurs from a fall onto a dorsiflexed wrist. The landing pushes the lunate out of place within the proximal row of carpal bones, towards the palmar surface of the wrist, which can result in compression of the median nerve. 
  • Compression of the median nerve is associated with carpal tunnel syndrome, therefore carpel tunnel syndrome can result in an individual that has suffered dislocation of the lunate. 

Kienbock's Disease

  • Also known as avascular necrosis of the lunate
  • This disease involves the collapse of the lunate due to vascular insufficient and avascular necrosis, as a result of fracture of the lunate. 
  • Disruption of blood supply to the lunate, repetitive compression of the lunate, between the distal radius and capitate and vascular impairment are associated risk factors to Kienbock's Disease
  • Kienbock's disease is felt as a stiffness, swelling and tenderness and the lunate itself,  as well as pain that radiates up the forearm. Limit motion on dorsiflexion and weakness when gripping also result.
  • As the disease progresses, pain and weakness at the wrist increases, and the degenerative changes become severe and chronic
  • This disease can not only result from direct trauma to the lunate, but also skeletal variation; irregular lunate or ulna that is shorter than the radius, as well as in individuals with medical conditions associated with poor blood supply. 

What do you now now?

The blood supply entering the scaphoid bone, enters at the distal pole

  • That is correct: The blood supply enters at the distal pole, hence why as a result of fracture at the waist of the scaphoid, the proximal pole of the bone dies, as it is unable to receive any source of bloody supply

In the radiocarpal joint, the lunate articulates with the distal end of radius, ulna and the capitate

  • The is not true. The ulna does not articulate with the bones in the wrist joint

Select three of which can occur as a result of fracture to the waist of the scaphoid bone

  • Avascular Necrosis
  • Carpal Tunnel Syndrome
  • Malunion
  • Non-Union
  • Kienbock's Disease

How long should a scaphoid fracture be immobilised in a cast for?

  • 3-4 Weeks
  • Never
  • 6-12 Weeks
  • 1-2 months

What are the four bones present in the proximal row of carpal bones?

  • Triquetral
  • Capitiate
  • Pisiform
  • Hamate
  • Trapezoid
  • Lunate
  • Scaphoid
  • Triquetrum

Which nerve is it that is compressed by anterior dislocation of the lunate?

  • Ulnar Nerve
  • Median Nerve
  • Superficial Branch of the Radial Nerve
  • Dorsal Branch of the Ulnar Nerve

Ligaments

Which image shows extension of the radoiocarpal joint

References

References

Reference List

Borgeskov, S., Christiansen, B., Kjær, A. and Balslev, I (1966). Fractures of the Carpal Bones. Acta Orthopaedica Scandinavica.37:3. pp. 276-287.

Compton, J. (1998). The anatomy of acute scaphoid fractures: A three-dimensional analysis of patterns. The Journal of Bone and Joint Surgery. 80:2. pp 218 - 224.

Stranding, S. and Gray, H (2016). Gray's Anatomy: The Anatomical Basis of Clinical Practice. 40th ed. Philadelphia: Elsevier Limited

McMinn, R., Gadda-Rosse, P., Hutchings, R. and Logan, B (1995). McMinn's functional & clinical anatomy. 1st ed. London [etc.]: Mosby. pp. 230 -236.

Moore, K., Dalley, A. and Agur, A (2014). Clinically oriented anatomy. 7th ed. Philadelphia: Wolters Kluwer. pp. 690 - 715. 

Nordic, M. and Frankel, V (2012). Basic biomechanics of the musculoskeletal system. 1st ed. Philadelphia: Wolters Kluwer/Lippincott Williams & Wilkins Health. pp. 364 - 396. 

Palastanga N & Soames R (2012). Anatomy and Human Movement: Structure and Function. 6th ed. Churchill Livingstone/Elsevier, Edinburgh. pp. 36 - 195.