Anatomy of the Flexor tendons:
The flexor tendons of the index,
middle,
ring and small fingers consist of the flexor digitorum superficialis (FDS) and the flexor digitorum profundus (FDP) and are responsible for flexing the joints of the hand and fingers.
The FDP is the only flexor of the distal interphalangeal joint (DIPJ).
The FDS tendons lie superficial (volar) to the FDP.
At the mid diaphysis of the proximal phalanx,
the FDS tendon splits in two and one passes medial to the FDP and the other passes lateral to it.
The FDS then inserts into the base of the middle phalanx and the FDP continues on distally to insert onto the base of the distal phalanx (Figure 1).
Anatomy of the Pulleys:
The pulleys are localized thickened foci of digital sheath that lie on the volar aspects of the flexor tendons and act to stabilize the tendons.
The fingers contain five annular and three cruciate pulleys (1,2) (Table 1).
The A or annular pulleys form bands with a semicircular arrangement.
The C pulleys pattern is different with a crisscross or cruciate pattern (Figure 2).
Pulley
|
Attachment
|
A1
|
Base of PP and MCPJ
|
A2
|
Proximal and mid diaphysis of PP
|
A3
|
Volar plate of PIPJ
|
A4
|
Mid diaphysis of Middle phalanx
|
A5
|
Volar plate of DIPJ
|
C1
|
Located between A2 and A3 pulleys at volar PIPJ
|
C2
|
Located between A3 and A4 pulleys immediately distal to the PIP J.
|
C3
|
Located between A4 and A5 pulleys immediately proximal to the DIPJ.
|
Table 1. Table showing the A and C pulleys and their attachment on the volar aspect of the flexor tendons of the fingers.
Proximal phalanx (PP),
metacarpalphalangeal joint (MCPJ),
proximal interphalangeal joint (PIPJ),
distal interphalangeal joint (DIPJ).
Ultrasound technique and Normal Sonographic appearances of the flexor tendons and pulleys.
The FDS and FDP are easily discernable separately in the palm.
Just proximal to the metacarpal head,
the tendons enter the common tendon sheath and distal to this,
it is more difficult to separate them and they can appear as a single tendon.
Simple maneuvers such as fixing the PIPJ and moving the DIPJ moves the FDP tendon in isolation and the two tendons can be better identified separately.
Imaging in the short axis (axial) plane best identifies the two separate tendons (Figure 3 and 4).
The annular pulleys are best identified in the long axis plane.
The A1 pulley is a seen as a thin hypoechoic band measuring approximately 5-6mm on the volar aspect of the flexor tendon (Figure 5).
Distal to this,
the A2 pulley lies in the mid aspect of the proximal phalanx and has a similar appearance to the A1 pulley (Figure 6).
Its integrity is confirmed on ultrasound by an attachment of the underlying flexor tendon to the proximal phalanx.
The remainder of the pulleys have similar appearences to the A1 and A2 pulleys.
Anatomy of the Extensor tendons:
The anatomy of the extensor tendons is more complex than the flexor tendons.
At the level of the MCP joints the extensor tendons are held in place by a strong retinaculum called the extensor expansion or hood (figure 7).
Distally,
the extensor expansions join to form lateral bands that coalesce with the central band of the extensor tendon and insert at the base of the middle phalanx.
At the level of the distal aspect of the proximal phalanx,
the extensor tendon divides into three.
A central band which joins with lateral bands from the extensor expansion and inserts at the base of the middle phalanx.
And then two slips,
medially and laterally which rejoin distally and insert as a single tendon at the base of the distal phalanx.
(Figure 7).
Songraphicallly,
as the extensor tendons are thinner than the flexor tendons they can be more difficult to evaluate.
Similar to other tendons in the body,
it displays an echogenic fibrillar pattern (Figure 8 and 9).
Pathology of the Flexor tendons:
Tenosynovitis:
Tenosynovitis is characterized by distension of the synovial sheath around a tendon with fluid.
The fluid appears anechoic on ultrasound.
Possible causes of this include,
degenerative ,
trauma,
inflammation,
infection or crystal deposition. There is an increase in vascularity when associated with inflammation (Figure 10).
Tendon rupture:
Rupture of the FDP tendon is more common than the FDS tendon and tends to occur just proximal to its insertion into the base of the distal phalanx.
It most commonly affects the index finger and is known as jersey or rugby finger.
The FDP avulsion injuries were originally classified by Leddy and Packer (3) where Type I-III were described.
Since then,
three more subtypes have been described (4-6).
Type of FDP Injury
|
Details
|
Type I
|
Tendon retraction into the palm.
Associated tear of the vincular blood supply.
Urgent surgical repair needed.
|
Type II
|
Tendon retraction to the level of the PIP joint .
Associated with some preserved vascular perfusion.
|
Type III
|
Avulsion fractures which prevent tendon retraction to the A4 pulley.
|
Type IV
|
Avulsion fracture where the avulsed fragment does not remain attached to the torn FDP tendon.
|
Type V
|
Avulsion fractures with and extraarticular (type 5a) or intraarticular (type 5b) fracture of the proximal phalanx.
|
Type VI
|
Open avulsion fractures with a lost fracture fragment.
|
Table 2. Table showing the classification of FDP injuries and how to recognize them.
Type I-III were described by Leddy and Packer (3).
Active flexion of the finger under ultrasound can help to in the differentiation between partial and full thickness tears (Figure 11).
Ultrasound is also useful to identify the proximal and distal stumps of the tendon in the preoperative phase.
By marking the two ends,
surgical exploration can be minimized.
Pulley fibroma:
Pulley fibromas most readily involve the A1 pulley at the level of the MCP joint.
It is thought the cause of the fibromas may be due to chronic friction.
With time,
a nodule develops in the flexor tendon just distal to the A1 pulley due to repeated friction.
Initially flexion of the finger results in clicking in and out from under the pulley fibroma .
Eventually,
a trigger finger results which is due to an inability of the nodule to pass under the pulley fibroma after finger flexion.
On ultrasound,
the fibromas are variable in appearance but tend to be hypoechoic and well demarcated.
There is no increase in vascularity (Figure 12)
Pulley rupture:
Pulley ruptures are most commonly seen in rock climbers and the A2 and A4 pulleys are most likely to be affected.
Normally,
the flexor tendons are held close to the phalanges by the pulleys.
In the case of a single pulley rupture,
the tendon moves 3-5mm in a volar direction away from the phalanx and this is called bowstringing (8,9) (Figure 13).
It is believed that the A2 pulley most commonly ruptures first and then progresses to the A4 pulley.
If multiple contiguous pulleys are torn a gap of up to 5-8mm can be seen in forced flexion (8,9) The A1 pulley very rarely ruptures and injuries of the C pulleys are also not common.
Partial pulley tears may not result in bowstringing.
Pulley ganglion.
Pulley ganglion most commonly occur in the A2 pulley and tend to arise from the proximal or distal ends of the pulley.
Under ultrasound,
the ganglia are usually very small,
well demarcated and hypoechoic in appearance (Figure 14 ).
If symptomatic,
ultrasound guided aspiration can be performed to help alleviate symptoms.
Pathology of the Extensor Tendons:
Paratenonopathy: As the extensor tendons do not have a true synovial sheath,
paratenonopathy rather than tenosynovitis occurs.
This tends to be in the presence of a foreign body or penetrating trauma.
Central slip rupture:
At the base of the middle phalanx the central slip joins with the two lateral slips (see above) and inserts at this point. A penetrating injury or hyperflexion can result in tendon rupture.
Plain radiographs can make the diagnosis there is an associated bony avulsion.
The rupture results in the classic boutonniere deformity; displacement of the lateral slips,
proximal retraction of the central slip and subluxation of the PIPJ.
In the absence of an avulsion,
ultrasound appearences show loss of tendon continuity and loss of the normal fibrillar pattern and a hypoechoic mass.
Mallet Finger
Mallet finger is due to avulsion of the insertion of the extensor tendon at the distal phalanx.
If associated with an avulsion injury,
diagnosis can be made on plain radiograph.
If not,
ultrasound demonstrates a tear in the tendon either at the attachment or just proximal to it.
There may also be some mild proximal retraction.
Dorsal hood injury.
Injuries of the dorsal hood occur at the metacarpal heads.
Clinically,
there is subluxation of the extensor tendon and often times ultrasound is not required to make the diagnosis.
If any question about the diagnosis,
scanning in the short axis plane while the patient is making a fist will display a subluxed extensor tendon and a tear in the dorsal hood may also be seen (Figure 15) .
This injury is also known as boxers knuckle.
Interventions:
Flexor tendon sheath:
Under ultrasound guidance,
fluid in the tendon sheath can be aspirated for both diagnostic and therapeutic purposes. Corticosteroid injection can also be performed.
An in view approach in the long or short axis is recommended and ideally with a small footprint probe.
Pulley Fibroma.
Treatment of a pulley fibroma involving disruption of the fibroma and corticosteroid injection under ultrasound guidance can be of great benefit to the patient.
Ideally a short footprint probe is used and placed in the long axis position overlying the fibroma.
The needle approach can be either proximally or distally in the long axis approach.
A bend in the needle can help the accuracy and ease of the procedure.
Following cleaning and anesthetic administration,
several passes in and out of the pulley can help divide the fibroma.