Abstract
Background: The pathogenesis of lateral epicondylitis remains
unclear. Our purpose was to study the anatomy of the lateral aspect of the
elbow under static and dynamic conditions in order to identify bone-to-tendon
and tendon-to-tendon contact or rubbing that might cause abrasion of the
tissues.
Methods: Eighty-five cadaveric elbows were examined to determine
details related to the bone structure and musculotendinous origins. We
identified the relative positions of the musculotendinous units and the
underlying bone when the elbow was in different degrees of flexion. We also
recorded the contact between the extensor carpi radialis brevis and the
lateral edge of the capitellum as elbow motion occurred, and we sought to
identify the areas of the capitellum and extensor carpi radialis brevis where
contact occurs.
Results: The average site of origin of the extensor carpi radialis
brevis on the humerus lay slightly medial and superior to the outer edge of
the capitellum. As the elbow was extended, the undersurface of the extensor
carpi radialis brevis rubbed against the lateral edge of the capitellum while
the extensor carpi radialis longus compressed the brevis against the
underlying bone.
Conclusions: The extensor carpi radialis brevis tendon has a unique
anatomic location that makes its undersurface vulnerable to contact and
abrasion against the lateral edge of the capitellum during elbow motion.
Clinical Relevance: This information may help us to understand the
pathomechanics of lateral epicondylitis and provide a better rationale for
operative and nonoperative treatment.
The pathogenesis of lateral epicondylitis (tennis elbow) remains unclear.
Currently, the most popular theory is that the condition results from repeated
contraction of the wrist extensor muscles—especially the extensor carpi
radialis brevis—causing microscopic tendon tears that progress to the
degenerative condition of
tendinosis1-5.
Attention has been directed primarily at internal stress from tension forces
on the
tendon3,6,7,
with little research addressing the anatomic relationships between the
extensor origins and the underlying osseous prominences at the elbow as a
possible source of injury leading to tendon attrition and eventual
rupture.
Several observations suggest that the anatomy and kinematics around the
elbow might be important in the occurrence of tennis elbow. First, the
pathological changes occur at a consistent location in the common extensor
tendon origin rather than at other locations along the tendon that are also
under longitudinal internal
stress1,5,7-11.
Second, evidence suggests that healthy tendons do not rupture in their
substance; instead, excessive tension causes either disruption at the
musculotendinous junction or avulsion of a fragment of
bone12. Third, the
symptoms of tennis elbow can be elicited by elbow motion, specifically elbow
extension, regardless of the position of the
wrist10,11,13,14,
or are more prominent with the elbow
extended3,10,14,15.
Finally, in addition to other effects, the preferred surgical procedures
alleviate symptoms of tennis elbow by changing the structural arrangement of
tendon and bone at the elbow by either cutting or excising portions of the
extensor
origin1,2,4,6,7,9,11,15,16.
The existing theories do not explain these observations; therefore, we
examined elbow anatomy to see if interaction such as contact, rubbing, or
pressure between tendons or between tendons and bone during elbow motion might
contribute to the cause of lateral epicondylitis. In 1985, Briggs and
Elliott17 reported
the findings of a static anatomical study of the origin of the extensor carpi
radialis brevis. They concluded that "tennis elbow is primarily a
mechanically-induced condition" and reported the tendon is
"subject to shearing stress in all movements of the forearm, especially
those involving power at the wrist." We are not aware of other similar
anatomic studies or studies of the dynamic relationship of the extensor carpi
radialis brevis tendon to adjacent structures. However, attritional tendon
damage elsewhere has been extensively
studied12,18.
We report our findings on the basic static anatomy of the lateral aspect of
the elbow, the relationships of the structures at different positions of elbow
flexion and extension, and the interaction (kinematics) of the structures
during elbow motion.
Static Anatomy: Osteology
The Department of Anatomy and Cell Biology at the University of North Texas
Health Science Center allowed us access to sixty embalmed (formalin perfusion
plus immersion) cadavers. The cadavers were hermetically sealed in plastic
bags and were stored at 44°F (6.7°C) until needed; they were then kept
in stainless steel tanks at 65°F (18.3°C). No elbow had been
previously dissected, and no elbow showed evidence of an old injury or
surgery. The average age of the donors at the time of death had been 73.6
years (range, fifty-three to eighty-three years); thirty specimens were from
male donors, and thirty were from female donors.
To determine the size and locations of bone prominences and tendon origins,
we made two series of observations. First, we performed an indirect
preliminary photographic study to understand more precisely the locations,
sizes, and variations of bone and tendon anatomy. Next, we directly measured
the same sizes and locations.
For the preliminary photographic study, we removed all of the soft tissue
from twenty cadaveric humeri after etching and marking the origin of the
extensor carpi radialis longus, the isolated origin of the extensor carpi
radialis brevis, and the most prominent point of the lateral epicondyle. All
dissections were done with use of 3.5-power loupe magnification and were
performed from distal to proximal so that the individual muscles could be
isolated. The humeri were stabilized in a frame holding a digital camera at
fixed, reproducible orthogonal angles 33 cm (13 in) from the specimen, and
pictures were made from directly anterior and directly lateral to the distal
part of the humerus at 90°; each photograph was centered on the edges of
the capitellum. A ruler was placed next to the humerus for each photograph to
control for magnification. We used Adobe Photoshop Elements 2.0 software
(Adobe Systems, San Jose, California) to analyze and measure the bone anatomy
and the location of the tendon origins with use of the axes of the superior
and lateral edges of the capitellum (see Appendix).
For the direct measurement study, forty additional elbows were dissected in
the same way. We placed a transparent square on the superior and lateral edges
of the capitellum and made measurements of the location of the same bone and
tendon structures from those axes with use of an electronic caliper that was
accurate to 0.1 mm (see Appendix).
Positional Anatomy: Anatomic Relationships with Differing Elbow
Positions
We studied the relationship of musculotendinous units to bone and to each
other with the elbow at various positions of flexion (90°, 45°,
30°, and 0°). We observed and recorded the positional relationships of
the capitellum, extensor carpi radialis longus, and extensor carpi radialis
brevis under three conditions: with the extensor carpi radialis longus and
extensor carpi radialis brevis together (Condition 1), with the extensor carpi
radialis longus alone (Condition 2), and with the extensor carpi radialis
brevis alone (Condition 3). A total of six cadaveric elbows were used. All six
were used to observe Condition 1, and then the six elbows were divided into
two groups of three elbows each. One of these groups was used for observation
of Condition 2, and the other was used for observation of Condition 3. We
prepared the extremities by removing the brachioradialis, extensor digitorum
communis, extensor carpi ulnaris, and extensor digiti quinti proprius and by
making a window in the elbow capsule over the capitellum. We left the extensor
carpi radialis longus and extensor carpi radialis brevis attached to their
humeral origins but cut their other soft-tissue attachments, including their
distal insertions and all connections between them. The extensor carpi
radialis longus and extensor carpi radialis brevis tendons were then replaced
into the second dorsal wrist compartment and were placed under 6 lb (2.7 kg)
of tension. Each extremity was mounted in a metal frame that fixed the humerus
but allowed elbow motion. Mountings for a digital camera were fixed to the
frame directly anterior and directly lateral to the distal part of the
humerus. Photographs were made in the frontal and sagittal planes, 20 cm (8
in) from the specimen with the elbow at each selected position. To measure
lateral tendon displacement, we placed reference marks on the capitellum and
on the medial edge of the musculotendinous unit. The frontal photographs were
analyzed for lateral movement relative to the mark on the capitellum, and the
sagittal photographs were analyzed for evidence of bone and tendon contact and
tendon distortion.
Injection Study
To further examine the relationships at different elbow positions, we
isolated and elevated the entire intact extensor mechanism to the humeral
origin in three additional elbows. The musculotendinous units were not
separated, so that the normal relationships would be maintained. We drilled a
hole transversely across the distal part of the humerus, starting at the
lateral surface of the proximal aspect of the capitellum and exiting through
the medial epicondyle. A 2-mm cannula was passed through the hole and was left
protruding medially but was placed flush with the capitellum laterally. The
wrist extensor muscles were put back into the normal wrist compartments and
were placed under tension. The elbow was then sequentially positioned at
90°, 45°, 30°, and 0° (full extension). At each stop, the
undersurface of the extensor mechanism was marked with methylene blue dye
using a 25-gauge 3.5-in (9-cm)-long needle that was introduced through the
cannula. The extensors were held under 6 lb (2.7 kg) of tension throughout
elbow motion and were allowed to follow their normal course. After the four
injections were made, the extensor mechanisms were reflected and examined.
Dynamic Anatomy: Tendon and Bone Relationships with Elbow Motion
The positional studies confirmed contact between the tendons themselves and
demonstrated tendon displacement and distortion from contact with bone, but
the usual site of pathology—the underside of the conjoined
tendon—could not be
seen1,5,7,9-11.
In order to analyze the kinematics of the deep fibers of the extensor carpi
radialis brevis during elbow motion, we removed all extensor muscles and
tendons, leaving just the elbow joint intact except for an anterior capsular
window. A single size-0 nylon suture was used to represent the deep fibers of
the extensor carpi radialis brevis, which could not be seen with the soft
tissue intact. The suture was stretched from the medial aspect of the etched
origin of the extensor carpi radialis brevis to the second compartment at the
wrist and was kept under constant 0.7 kg of tension. The elbow was moved
through a range of motion, and the kinematics of the suture was observed.
Initially, we used ten adult cadaver elbows; five were from male donors and
five were from female donors, and five were from the left side and five were
from the right side. A mark was placed on the capitellum, and another mark was
placed on the suture, 1 cm distal to its emergence from the epicondyle. The
elbow was then moved through a range of motion and was stopped at 90°,
75°, 60°, 45°, 30°, 15°, and full extension. At each elbow
position, the distance from the capitellar reference mark to the suture was
measured with electronic calipers. The index point (designated 0 displacement)
was defined as the measurement that was made with the elbow at 90° and the
forearm in neutral rotation. Measurements were made in maximum supination,
neutral, and maximum pronation (see Appendix).
Six additional cadaver elbows were used to perform the same observations
with the capitellum intact and subsequently with the lateral third of the
capitellum removed without jeopardizing elbow stability. The same measurements
were made with use of these elbows but only in neutral rotation.
Static Anatomy
Osseous Anatomy
In its distal fourth, the humeral shaft flattens and spreads to become
spatula-shaped, making a base for the elbow joint and muscular attachments.
The lateral flair of the bone from the shaft to the lateral condyle averaged
21.8° (range, 14° to 30°), resulting in the epicondyle being an
average of 2.5 cm (range, 1.8 to 3.2 cm) lateral to the central axis of the
humeral shaft. The distal part of the humerus tilted forward an average of
34.1° (range, 26.2° to 48°), which placed the anterior edge of the
capitellum an average of 2.6 cm (range, 2.1 to 3.3 cm) anterior to the axis of
the shaft and the muscle origins (Fig.
1).
We found the anatomical features of the distal part of the
humerus—the size and shape of the capitellum and lateral epicondyle and
the locations of muscle origins—to be quite variable as indicated by the
wide ranges in measured values. Figure
1 shows reference bone and tendon locations, with the inset
detailing the extensor carpi radialis brevis origin measurements. From the
anterior view, the lateral condyle (capitellum) approximated the shape of a
hemisphere averaging 2.3 cm (range, 1.7 to 2.7 cm) in diameter. The epicondyle
protruded an average of 4.8 mm (range, 0.7 to 8.7 mm) lateral to the outer
edge of the capitellum at its widest point. The most prominent lateral
projection—the most easily palpable point of the
epicondyle—usually lay just proximal (average, 0.7 mm; range, —2.7
to 6.1 mm) to the proximal edge of the capitellum. As shown in
Figure 1, when viewed from the
lateral side, the anterior edge of the capitellum was an average of 2.6 cm
(range, 2.1 to 3.3 cm) anterior to the lateral humeral axis. When viewed
end-on, along the axis of the humerus (Fig.
1), the lateral edge of the capitellum flared from its base to
protrude an average of 1 mm (range, 0 to 4.6 mm), or 6.8° (range, 0°
to 18°). These observations demonstrate that much of the lateral
epicondyle is medial and posterior to the outer, anterior edge of the
capitellum.
Musculotendinous Anatomy
The muscles of the lateral aspect of the elbow are the brachioradialis,
extensor carpi radialis longus, extensor carpi radialis brevis, extensor
digitorum communis, extensor carpi ulnaris, and supinator. The extensor carpi
radialis brevis, extensor digitorum communis, and extensor carpi ulnaris were
seen to attach to the humerus together as a strong, identifiable conjoined
tendon. The extensor digiti quinti proprius blended with the extensor
digitorum communis in the forearm and was difficult to distinguish within the
conjoined tendon. All of these structures—the brachioradialis, extensor
carpi radialis longus, conjoined tendon, and supinator—blended together
at the elbow and attached to the humerus as a merged extensor mechanism
(Fig. 2). These structures are
distinct only in the
forearm10,14,19,20.
The extensor carpi radialis longus and the extensor carpi radialis brevis
were intimately blended and could not be distinguished from one another in the
proximal part of the forearm; however, if the musculotendinous units were
separated in the distal part of the forearm, the separation could be extended
proximally to the elbow if done with care and the use of magnification. Along
the inferior edge of the extensor carpi radialis longus, there was a 3 to
5-mm-wide fascial band enclosing it and providing continuous attachments for
the fusiform, bitendinous extensor carpi radialis brevis. The line of
separation usually led directly to the most prominent point of the
epicondyle.
The extensor carpi radialis longus attached along the supracondylar ridge
as a muscle14,
without tendon intervening between it and bone. The distal extent of its
insertion averaged 0.6 cm (range, 0 to 1.3 cm) proximal to the superior edge
of the capitellum. The muscle then extended proximally an average of 3.5 cm
(range, 2.3 to 5.3 cm) so that its body lay in an area directly proximal to
the capitellum. When viewed laterally, the extensor carpi radialis longus
origin was seen to be in line with the longitudinal axis of the humerus. This
put the extensor carpi radialis longus origin posterior and medial to the
epicondyle, the capitellum, and the extensor carpi radialis brevis origin.
The origin of the extensor carpi radialis brevis was one of our most
interesting findings. The purely
tendinous14
extensor carpi radialis brevis attachment lay deep and superior within the
stout tendon shared with the extensor digitorum communis and extensor carpi
ulnaris. This arrangement placed the extensor carpi radialis brevis adjacent
to the capitellum and covered by the extensor digitorum communis tendon in
addition to the extensor carpi radialis longus muscle mentioned above.
We measured the location of the extensor carpi radialis brevis origin
(Fig. 1, inset) in relation to
the lateral and superior margins of the capitellum. In relation to the lateral
margin of the capitellum, we found the medial edge of the extensor carpi
radialis brevis to lie an average of 1.7 mm medial (range, 5.9 mm medial to
1.2 mm lateral) to the outer edge of the capitellum; its lateral edge averaged
3.8 mm (range, 0.8 to 6.3 mm) laterally. The medial aspect of the extensor
carpi radialis brevis insertion was medial to the outer edge of the capitellum
in thirty-two of the forty directly measured specimens.
The proximal edge of the extensor carpi radialis brevis was an average of
1.2 mm proximal to the superior margin of the capitellum (range, 3.7 mm distal
to the superior margin to 8.0 mm proximal to the superior margin). The distal
extent of the extensor carpi radialis brevis origin was an average of 6 mm
distal to the superior margin of the capitellum (range, 12.9 distal to the
superior margin to 10.4 mm proximal to the superior margin). The most distal
extent of the extensor carpi radialis brevis origin was never more distal than
the middle of the capitellum and usually was well proximal to the middle of
the capitellum (the site of the collateral ligament attachment).
Positional Anatomy
Extensor Carpi Radialis Longus and Extensor Carpi Radialis Brevis
Together (Fig. 3)
With the elbow in 90° of flexion, the extensor carpi radialis longus
and extensor carpi radialis brevis took a straight-line course from their
humeral attachments to the second dorsal wrist compartment. There was no
contact with the elbow, and the extensor carpi radialis longus and extensor
carpi radialis brevis lay adjacent to each other. With the elbow in 45° of
flexion, the muscle showed evidence of contact with the underlying structures.
The muscle flattened when viewed anteriorly and bowed when viewed laterally.
As extension continued, the medial edge of the extensor carpi radialis longus
pressed more firmly on the condyle and was pushed laterally. In the set of six
observations involving both musculotendinous units, the extensor carpi
radialis longus moved an average of 6 mm laterally when the elbow moved from
flexion to full extension. When viewed laterally, the extensor carpi radialis
longus and the extensor carpi radialis brevis bowed over the capitellum and
epicondyle as extension progressed. With further extension, the extensor carpi
radialis longus stretched over, covered, and hid the proximal aspect of the
extensor carpi radialis brevis tendon in all specimens. We interpreted the
bowing and stretching of the extensor carpi radialis longus muscle as an
indication that it pressed on the underlying extensor carpi radialis brevis
and capitellum, thereby potentially increasing the abrasive forces.
Extensor Carpi Radialis Brevis Alone
(Fig. 4)
When the extensor carpi radialis longus was removed, the isolated extensor
carpi radialis brevis could be seen through the entire course of elbow motion.
At 90° of elbow flexion, it ran a straight course from the humerus to the
wrist, lying superior and lateral to the condyle. As extension progressed, the
tendon was positioned between the point of the epicondyle and the lateral edge
of the condyle, sliding against and being pushed laterally by the capitellum
by 1, 2, and 6 mm in our three specimens. When viewed from the side, at full
extension, the extensor carpi radialis brevis stretched and bowed notably over
the condyle and then, after sliding laterally, it bowed over the
epicondyle.
Injection Study
With the end of the cannula lying at the edge of the lateral aspect of the
capitellum, the four injections formed a line transverse to the direction of
the conjoined tendon fibers, 1 to 1.5 cm long on the underside of the muscle
unit, approximately 1 cm from the tendon attachment to the humerus
(Fig. 5). After the injections,
we separated the tendons to identify which part of the extensor mechanism
passed the capitellum during elbow motion. In all three specimens, the dye was
injected into the extensor carpi radialis brevis tendon; however, in two
specimens, the dye that was injected when the elbow was in full extension
stained some fibers of the extensor carpi radialis longus.
Dynamic Anatomy (Fig.
6 and Appendix)
In all ten elbows that were included in the initial dynamic study, the
suture contacted the lateral edge of the capitellum at 75° of elbow
flexion and was displaced laterally because of this contact with the
capitellum, with more than half of the suture displacement occurring in the
first 45° of elbow extension. Contact area and suture displacement varied
between cadavers as shown by the ranges of measurements. The capitellum
displaced the suture laterally by an average of 3.4 mm (range, 2 to 6.4 mm)
when the forearm was in neutral rotation. The total amount of displacement was
less with the forearm in supination (average, 2.4 mm; range, 1.1 to 5.2 mm)
and more with the forearm in pronation (average, 4.3 mm; range, 1.8 to 8.7 mm)
(Table I). When the capitellum
was partially resected, the suture was no longer displaced as far laterally as
it was with the capitellum intact. The final suture displacement without the
capitellum averaged 1.3 mm (range, 0 to 2.1 mm). The suture displacement
occurring in these specimens during the final 30° of extension was due to
contact with the radial head and forearm muscles.
In the first ten elbows that were examined with a suture representing the
extensor carpi radialis brevis, the suture stayed in contact with the
capitellum throughout the range of motion tested. In the second set of six
elbows, we found that the suture did not maintain contact in three elbows. In
two of these elbows, the suture was lifted off the capitellum as the elbow
reached full extension because of contact with the radial head or part of the
supinator muscle covering the neck of the radius, once at 45° and once at
30°. In the third elbow, the suture did not touch the capitellum at all in
neutral rotation because the extensor carpi radialis brevis origin was lateral
to the capitellum and the elbow was in valgus; in this elbow, the suture
contacted the capitellum starting at 65° when the forearm was in
pronation.
Once we demonstrated rubbing of the suture against bone, we made videos of
the suture contacting, being displaced laterally, and rubbing against the
capitellum. This is by far the most dramatic and convincing evidence that
extensor carpi radialis brevis abrades against the capitellum during elbow
motion (see Appendix).
We believe that our observations of the anatomy of the lateral aspect of
the elbow show that commonly there is considerable contact between the
undersurface of the extensor carpi radialis brevis and the lateral edge of the
capitellum. We also believe that this contact might be a factor leading to the
tendon injury called "tennis elbow," or lateral epicondylitis.
In our static anatomic study, the muscle and tendon origins were found to
lie posterior and medial to the prominences of the distal part of the humerus,
meaning that with elbow extension the muscles and tendons will contact and
abrade against bone. The extensor carpi radialis brevis tendon, being the
deepest part of the extensor mechanism and lying proximal and slightly medial
to the protruding capitellum, is in a position to be particularly subject to
abrasion. The static anatomy observations also showed that there is a great
deal of variation in the size and shape of the capitellum and in the location
of the tendon origins. These variations, of which we believe the location of
the extensor carpi radialis brevis to be the most important, may explain why
tendon injury develops in some people and not others.
The most common yet vague description of the site of damage in tennis elbow
is stated to be the origin of the extensor carpi radialis brevis. This has
been reported by some to be the junction of tendon with
bone4, but more
descriptions and illustrations in the literature place the injury in the
proximal segment of the extensor carpi radialis brevis and the adjacent
tendons1,5-11,14.
In the present study, injection markings on the undersurface of the extensor
mechanism indicated that the extensor mechanism slides transversely against
the lateral edge of the capitellum as the elbow extends and flexes. These
marks identified which part of the extensor mechanism opposes the capitellum.
They were approximately 1 cm distal to the extensor origin and corresponded
with the most commonly described location of damage in patients with tennis
elbow. The markings were primarily in the extensor carpi radialis brevis but
also into some fibers of the extensor carpi radialis longus. We conclude that
the structures involved in contact and wear are the extensor mechanism
(specifically, the extensor carpi radialis brevis, 1 cm distal to its osseous
origin) and the lateral edge of the capitellum, and we believe that this wear
leads to tendon abrasion as an initial step in the cause of tennis elbow.
Dramatic bowing and stretching of the tendons over the epicondyle and the
capitellum occur with the elbow in full extension. It is unclear whether this
tension plays a role in augmenting tendon damage; however, it may be the
reason that pain often increases with elbow extension.
Having shown that the extensor carpi radialis brevis rubs on the lateral
condyle during elbow motion leaves the question of how this relates to tennis
elbow. As tendons can withstand tensile forces larger than can be exerted by
their muscles or sustained by
bone12, tendon
rupture suggests that there is preexisting pathology before mechanical
overload can result in tearing of the
tendon12,18.
The wear or abrasion of the proximal 1 to 2 cm of the extensor carpi radialis
brevis resulting from rubbing on the lateral condyle could cause attritional
damage.
While our study was undertaken solely to identify possible anatomic
relationships that might contribute to the cause of tennis elbow, it might
also have implications for the surgical treatment of this condition. If tennis
elbow is, in fact, a developing or impending attritional tendon rupture, then
relieving this contact seems to be the logical treatment. The easiest way to
accomplish this would be to cut and/or excise those soft tissues rubbing on
bone—the extensor carpi radialis brevis, the distal fascia of the
extensor carpi radialis longus, and part of the extensor digitorum communis.
Tendon release from the humerus distal to the proximal third of the capitellum
is probably not needed, and the lateral ligament must be
protected7,14,19,21.
There is good evidence that the extensor carpi radialis brevis gets adequate
support from extensive fascial attachment to prevent its retraction
distally5,9,19,22
and that proximal release is safe. This proposal is in agreement with the
abundance of favorable results obtained in association with both open and
arthroscopic procedures to detach and realign the extensor
origin1,2,4,6,7,9,15.
Failure to release or excise an adequate amount of tissue to relieve all
abrasion may explain the failure of some surgical procedures.
We agree with Briggs and
Elliott17 that
tennis elbow is a mechanically induced condition. However, we believe that
tendon injury resulting from lateral abrasion against bone during elbow motion
is an important factor in its cause. This injury under the stress of
longitudinal tension leads to tearing of the tendon as seen in other
attritional tendon ruptures. Hopefully this anatomic study will lead to more
effective treatment and prevention of this painful and disabling
condition.
Photographs showing the experimental setup are available with the
electronic versions of this article, on our web site at jbjs.org (go to the
article citation and click on "Supplementary Material") and on our
quarterly CD-ROM (call our subscription department, at 781-449-9780, to order
the CD-ROM). A video demonstrating the extensor carpi radialis brevis rubbing
on the capitellum is also available on our web site. ?
Note: The authors thank the members of the Department of Anatomy
and Cell Biology at the University of North Texas Health Science Center at
Fort Worth, Texas, for their assistance with this project.
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