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Articulated External Fixators

Ovid: Master Techniques in Orthopaedic Surgery: The Elbow

Editors: Morrey, Bernard F.
Title: Master Techniques in Orthopaedic Surgery: The Elbow, 2nd Edition
> Table of Contents > Part II – Trauma > 8 – Articulated External Fixators

8
Articulated External Fixators
Bernard F. Morrey M.D.1
Robert N. Hotchkiss M.D.2
1 Mayo Medical School and Department of Orthopaedics, Mayo Clinic and Mayo Foundation, Rochester, Minnesota.
2 Hand Service, Hospital for Special Surgery; and Clinical Surgery (Orthopaedics), Weill Medical College, New York, New York.
Two principal goals are simultaneously sought with use
of the articulated external fixator: (a) to allow active or passive
motion, (b) to protect the articular surface and the collateral
ligaments.
Indications/Contraindications
Protection of the articular surface is required in the following situations:
  • Coronoid fracture with or without fixation.
  • Olecranon fracture with tenuous fixation.
  • Distal humeral articular fractures.
  • Unstable ulnohumeral joint after acute collateral ligament disruption.
  • Combination of instability with any of the preceding fractures (complex instability).
Reconstruction
As an adjunct for individuals undergoing release for a stiff elbow.
This is the most common in the posttraumatic condition
but occasionally is used with inflammatory stiffness. The use of
distraction is generally indicated in these circumstances.
  • There has been a significant amount of dissection, suggesting that maintaining the intraoperative motion will be difficult.
  • If the pathology has modified the joint
    contour, requiring refashioning of the joint surface, with or without
    an interposition membrane.
  • When an interposition procedure is performed.
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  • If the collateral ligament has been reconstructed or repaired in association with the release.
For the most part the following conditions are relative contraindications.
  • Inexperience with the use of external
    fixation devices is considered a relative contraindication. Application
    of this device is technically demanding and requires accurate placement
    of the skeletal pins.
  • If uncertainty exists with regard to the
    anatomic location of the neurovascular structures due to posttraumatic
    destruction of the joint, the distraction device should be used only
    with extreme caution. (The pins, under these circumstances, may be
    inserted under direct vision.)
  • Local sepsis is a relative contraindication to the application of this device.
  • The presence of some fracture fixation devices in the distal humerus or proximal ulna.
  • Preemptive medical condition (e.g., severe osteoporosis).
Preoperative Planning
Reliable identification of the axis of rotation and
rigid skeletal fixation can be obtained by use of an articular device
that replicates the axis of rotation.
Skeletal fixation on the ulna with the Dynamic Joint
Distractor (DJD) II allows protection or neutralization of the
articular surface for a variety of clinical circumstances.
Figure 8-1.
Half- and full-pin fixation with uni- or bilateral frame application
offers significant application flexibility and clinical uses for the
DJD II.

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Motion in flexion/extension is allowed without
encumbrance, particularly in both the articular surface and the
collateral ligaments.
Options: The DJD II may be
used in either a monolateral or a bilateral configuration. This allows
a great deal of flexibility of use and indications (Fig. 8-1).
Surgery
The patient is placed supine with a sandbag under the
scapula. The arm is draped free with a nonsterile tourniquet and is
brought across the chest (Fig. 8-2). The elbow
is exposed according to the pathology present. Regardless of the
exposure or pathology, the essential landmarks for axis pin placement
are critical.
On the lateral aspect of the capitellum, a tubercle is
present at the site of the origin of the lateral collateral ligament.
This tubercle also represents the geometric center of curvature of the
capitellum, which is the site of the flexion axis of the elbow and is
the point through which a 3-mm Apex humeral reference pin will pass (Fig. 8-3).
On the medial aspect of the distal humerus, the axis of
rotation lies just anterior and inferior to the medial epicondyle. The
axis pin is placed in this region or slightly anterior and proximal to
this location (Fig. 8-3). This represents a safe zone relative to the ulnar nerve.
Hence these two loci are used to position the humeral (axis) reference pin guide (Fig. 8-3).
For a lateral frame application a 3-mm Apex humeral reference pin is
drilled or tapped 10 to 20 mm into the distal humerus. If there is
gross distortion, the center of the trochlea is the desired location
for the pin, since the ulna rotates on the humerus and rotation on the
capitellum is a secondary feature. If a medial frame is to be applied,
the ulnar nerve is identified and protected at the time of insertion of
the 3-mm Apex humeral reference pin.
Articular Fracture
The articular fracture is approached according to surgeon preference, the specific pathology, and the treatment goals (see Chapter 1). Olecranon fractures are easily exposed and the fixator is readily applied (Chapter 6). Fractures involving the coronoid require more extensive exposures, as described later for the release of the stiff elbow (Chapter 7). Distal humeral fractures may be treated by exposure with olecranon osteotomy

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or a triceps reflection technique (Chapters 1 and 4).
If the fracture fixation device(s) or collateral ligament reattachment
precludes the introduction of a 3-mm axis pin, a small Kirschner wire
is inserted in a manner to replicate the axis of rotation.

Figure 8-2. The patient is supine, the arm is brought across the chest, and most commonly a posterior skin incision is employed.
Figure 8-3.
The axis landmark on the distal humerus laterally is the axis of the
projected center of the capitellum. Medially, the axis is at the
anterior inferior aspect of the medial epicondyle.
The Stiff Elbow
If the elbow is being treated for stiffness, the
previous incision is entered, and an extensile lateral Kocher-type
joint release is used.
Typically, the triceps is reflected from the tip of the
olecranon (Mayo Modified Kocher). However, in some instances, such as
when elbow flexion is normal, the triceps may be left intact. The
capsule is exposed by releasing the common extensor tendon. If the
pathology is extrinsic to the joint, the anterior capsule is excised
but the lateral collateral ligament is preserved. If the joint is
abnormal and is to be altered, as with an interposition arthroplasty,
the lateral collateral ligament is elevated as a flap of tissue from
its origin at the lateral condyle. This is tagged and reflected
distally, providing an extensive exposure (Fig. 8-4), but must be repaired and reattached at closure.
When the pathology involves a joint surface that
requires an extensive dissection, the identification and protection of
the ulnar nerve is necessary. Ideally, a single posterior incision is
utilized, and a subcutaneous dissection is carried out to the medial
aspect of the triceps. If a previous Kocher skin incision has been
placed laterally, ulnar nerve exposure is accomplished through a
supplemental medial incision. In any event, the ulnar nerve is
identified but is usually not translocated anteriorly. Instead, it is
simply protected, first during the capsular dissection and later at the
time of axis pin placement. If ulnar nerve symptoms are present, then
the nerve is decompressed with definitive management, according to the
dictates of the pathology.
With the 3-mm Apex humeral reference pin in place, 2-mm
holes are made distal and proximal to the axis site for reattachment of
the lateral collateral ligament (Fig. 8-5).
Bunnell or Krachow sutures are placed through the radial (lateral)
ulnar collateral ligament and through the holes drilled through the
lateral column around the flexion pin.
Technique
To construct the frame, follow these steps:
  • Axis of rotation.
    Determine the axis of rotation external landmarks and place the humeral
    (axis) reference pin guide in line with the axis of rotation. The tip
    of the guide

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    is usually placed on the medial side while the pin guide on the lateral side as the axis pin enters laterally (Fig. 8-6).

    Figure 8-4.
    An extensile surgical exposure typically involves elevating the sleeve
    of extensor muscles and releasing the lateral collateral ligament.
  • Reference pin that acts as a guide during frame construction.
    Insert laterally the 3-mm-diameter self-drilling/self-tapping Apex pin
    through the humeral (axis) reference pin guide in the axis of rotation.
    For monolateral frame construction insert the pin to a depth of 15 to
    20 mm. For bilateral frames it is recommended to replace the 3-mm Apex
    humeral reference pin by a 3-mm smooth transfixing Apex pin that is
    inserted across the distal humerus (see “Bilateral Frame Option”).
    Note: The 3-mm pin is a reference pin and is the essential reference required to accurately

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    assemble the DJD II frame and to properly insert the humeral and ulnar pins. It will be removed after frame construction.

    Figure 8-5. A,B: Holes that are placed around the axis of rotation allow the ligament to be reattached with an osseous attachment.
    Figure 8-6. A,B:
    The C-guide with the sharp tip placed medially under direct vision
    allows accurate orientation of the axis reference pin even if placed
    only partially across the joint.
  • Remove the humeral (axis) reference pin guide.
  • Placement of the DJD II frame on the reference pin.
    The hollow-bored hinge of the DJD II is placed over the reference pin
    so that its hinge is exactly in the same axis of rotation as the
    natural axis of rotation of the elbow. Verify that the distraction
    device is fully compressed before frame construction.
  • Pin insertion.
    At this stage, depending on surgeon preference or features of the case,
    one may insert either the humeral or the ulnar pins. Humeral pin
    insertion is the author’s choice.
  • Humeral pin insertions.
    • Apply the 3-mm or 4-mm drill guide over the humeral rod so that pin guide is aligned with the lateral humerus (Fig. 8-7).
      Note: The 5-mm humeral rod is aligned to the anterior cortex of the humerus (INSERT).
    • The proximal humeral
      self-drilling/self-tapping Apex 4-mm (or 3-mm) pin is inserted into the
      lateral cortex of the humerus through the pin guide and engages the
      opposite cortex.
    • The pin guide is then removed.
    • The proximal pin is fixed to the humeral
      rod with a Hoffmann II Compact pin-to-rod coupling that is then
      tightened using a Hoffmann II Compact wrench (Fig. 8-8).
      Figure 8-7.
      The fixator is placed over the reference pin, and using the alignment
      guide a proximal half-pin is placed through the lateral and medial
      humeral cortices. The humeral arm of the fixator aligns with the
      anterior cortex of the humerus (INSERT).

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      Note: Hoffmann II Compact pin-to-rod couplings accept pins of both 3 and 4 mm in diameter.
    • Place the pin guide over the humeral rod more distally (closer to the hinge).
    • The second self-drilling/self-tapping Apex 4-mm (or 3-mm) pin is now inserted more distally through the pin guide (Fig. 8-9).
      Note: The pins need not
      necessarily be parallel. If a different pin insertion angulation is
      required to access a more desirable area on the humerus, slightly (5
      degrees) rotate the pin guide over the humeral rod until such a pin
      insertion area can be reached. By ensuring proper pin/rod distance, the
      system allows independent pin placement that is not coplanar (INSERT).
    • The pin guide is then removed from the humeral rod.
    • The distal pin is fixed to the humeral
      rod with a Hoffmann II Compact pin-to-rod coupling that is then
      tightened using a Hoffmann II Compact wrench.
  • Ulnar pin insertions.
    • According to the pin diameter (3 or 4
      mm), place the appropriate pin guide over the ulnar rod to access the
      lateral aspect of the ulna.
      Note: Three-millimeter pins are usually preferred, as the ulna diameter is smaller.
      Figure 8-8. The proximal pin is fixed to the humeral rod with the H2C coupling mechanism.
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    • The distal ulnar
      self-drilling/self-tapping Apex 3-mm (or 4-mm) pin is inserted into the
      lateral cortex through the pin guide and pierces the medial ulnar
      cortex (Fig. 8-10).
    • The pin guide is then removed.
    • The distal pin is fixed to the ulnar rod
      with a Hoffmann II Compact pin-to-rod coupling, which is then tightened
      using a Hoffmann Compact wrench.
    • Place the pin guide over the ulnar rod more proximally (i.e., between the distraction mechanism and the distal pin).
    • The proximal self-drilling/self-tapping Apex pin can now be inserted through the pin guide (Fig. 8-11).
      Note: As at the humerus, the
      pins are not necessarily parallel. If a different pin insertion
      angulation is required to reach a more adequate pin insertion area,
      slightly rotate the pin guide over the ulnar rod until such a pin
      insertion area can be reached. The system allows an independent pin
      placement (Fig. 8-9).
    • The ulnar pin guide is then removed.
    • The proximal pin is fixed to the ulnar
      rod with a Hoffmann II Compact pin-to-rod coupling, which is then
      tightened using a Hoffmann II Compact wrench.
    • If the indication requires the use of the proximal ulnar pin in the olecranon, it can be

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      inserted through the pin guide. This pin will be once again attached to
      the ulnar rod with a Hoffmann II Compact pin-to-rod coupling, which is
      then tightened.

      Figure 8-9. A,B:
      Using the pin guide, a second half-pin is placed across the proximal
      humerus distal to the first. If needed, a plus/minus 5 degrees of
      out-of-plane rotation may be introduced to provide better target as
      well as avoid soft-tissue injury (INSERT).
      Figure 8-10. A distal ulna half-pin is applied with the use of the pin guide. Typically, the 3-mm guide is used.
      Figure 8-11. After the distal pin has been coupled with the ulnar rod, a second pin is placed using the pin guide.
  • Reference pin removal. The 3-mm Apex humeral reference pin is then removed (Fig. 8-12).
  • Distraction.
    The ulna is separated from the humerus by turning the distraction screw
    using a Hoffmann II Compact wrench. Most commonly, 2- to 3-mm
    distraction is sufficient to accomplish the goals of the procedure (Fig. 8-13). Skin closure is usually deferred until the distraction is applied.
Bilateral Frame Option
If greater stability of the external fixator is desired,
a second half frame is applied over the smooth transfixing Apex pin on
the medial aspect. Independent half-pins are then applied on both the
humerus and the ulna as described in steps 6 and 7 (Fig. 8-1).
Medial Application
Exposure of the Joint—Stiff, Ankylosed, or Posttraumatic Joint
A “medial column” exposure of the elbow is most commonly
used, especially if ulnar nerve symptoms are present. The medial aspect
of the triceps is identified along with the ulnar nerve. The nerve is
not necessarily transposed unless appropriate for the case. The
intermuscular septum is identified proximal to the epicondyle and
followed anteriorly to the humerus. The soft tissues are elevated from
the distal humerus, and the pronator attachment is released

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from
the anterior superior aspect of the medial epicondyle. Elevating the
soft-tissue sleeve allows exposure of the anterior medial capsule (Fig. 8-14).

Figure 8-12. A,B: The reference (axis) pin is removed, leaving no interarticular fixative device.
Figure 8-13.
Using the hand wrench the distraction device is advanced 2 to 3 mm or
until the flexion arc occurs without evidence of articular contact.
To apply the DJD II fixator, place the humeral (axis)
reference pin guide in line with the axis of rotation. The guide stylus
is placed medially, and the point is placed laterally at the axis site
located at the lateral tubercle. Insert medially the 3-mm Apex humeral
reference pin (Fig. 8-15). The application
proceeds as with the lateral description. However, care must be
exercised to observe and protect the ulnar nerve and anterior
neuromuscular bundle at the time of humeral pin insertion. This is best
done by directly observing the entrance site of the percutaneous pins
at the humerus (Fig. 8-16).
For some acute or subacute fractures in which the elbow
is unstable, there is a tendency for the ulna to sublux posteriorly. In
these cases, the DJD II may be applied to neutralize this tendency. The
typical features of the application under these circumstances include
the following:
  • The use of fluoroscopy, so that the pins may be inserted percutaneously.
  • Insertion of the pins distal to the
    coronoid to avoid any fracture fixation that may be present, but also
    to apply the correct distal displacement vector to help accomplish
    elbow joint reduction.
The patient’s extremity is draped free, and a C-arm
fluoroscopic unit is also draped in a sterile fashion. It may be
difficult to palpate the lateral epicondyle if there is a significant

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amount
of swelling. Thus the location in the midpoint of the lateral
epicondyle is identified by AP and lateral projections, using a
hypodermic needle or Steinmann pin to identify the point of insertion.

Figure 8-14.
After identification of the medial intermuscular septum the pronator
origin and brachialis are swept from the anterior aspect of the distal
humerus.
Figure 8-15.
The axis pin is passed through the axis of rotation from medial to
lateral using the alignment guide and medial axis target area past the
inferior aspect of the medial epicondyle.
Figure 8-16. The humeral pins medially are most safely placed percutaneously under direct vision.
Figure 8-17. A,B: Percutaneous application directs the reference axis pin from the lateral epicondyle toward the medial epicondyle.
A 3-mm Apex pin used as the reference pin is placed
laterally and directed toward the point identified at the medial
epicondyle (Fig. 8-17). The DJD II is then
applied over the reference pin, and the humeral half-pins are inserted
percutaneously using the appropriate pin guide (see the preceding step
6) (Fig. 8-18). Hoffmann II Compact pin-to-rod
couplings are used to attach the humeral pins to the humeral rod of the
DJD II. The joint is reduced as able and the ulnar pins are then
applied with the appropriate pin guide. Ulnar pins are then attached to
the ulnar rod with Hoffmann II Compact pin-to-rod couplings.
Note: The joint is reduced
before the frame is secured; however, small adjustments to alignment
can be made by advancing the distraction mechanism (Fig. 8-19).
Compass Hinge
To use the Compass hinge, the frame is first assembled.
In most instances, the 150-mm ring size is the best. The geared
component is always medial; the knob is always posterior (Fig. 8-20).
When the frame has been placed and appropriately
assembled and adjusted, it will slide along the axis pin without
significant impingement or resistance (Fig. 8-21).
The clinician should make sure to allow for swelling in the
postoperative period, allowing at least 2 cm of clearance from the skin
to the hinge block at the time of surgery.
For release of contracture and removal of heterotopic
bone, the patient is placed in the supine position with the arm on a
radiolucent hand table. If the patient first requires a more

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extensive
exposure of the distal humerus for fracture or reconstructive
procedure, it may be useful to begin the operation with the arm over
the chest, using one of the more standard posterior approaches to the
elbow, either olecranon ostectomy or triceps-sparing exposure if
Bryan/Morrey or a Mayo Modified Kocher release (see Chapter 1).
In cases of gross instability, the prone position can be used; however,
exposure of the coronoid is quite difficult in this position.

Figure 8-18.
The proximal half-pin is inserted in a percutaneous fashion (of right
arm). Fluoroscopy is recommended for assistance as necessary.
Figure 8-19. After application of the apparatus, the joint is distracted according to the needs of the pathology.
Figure 8-20. The Compass hinge employs hemi-rings and a gear mechanism that allows free motion on applied force across the mechanism.
Figure 8-21. The preconstructed frame allows free slide of the axis pin.
As with the DJD, a single temporary axis pin can be
placed across the joint, or two half-pins can be inserted, one from the
medial and the other from the lateral aspect of the joint. The
alignment of the axis is crucial. It is important to take the time
necessary to achieve perfect placement of this pin for alignment of the
Compass hinge at the elbow. Anteroposterior and lateral radiographs
ensure adequate placement. Once the two pins are coincident, the frame
should still be easy to slide from medial to lateral, back and forth,
before securing the axis pin from the medial side (Fig. 8-22).
The principle in placing the humeral pin is to secure
the humerus in two planes, without impaling any of the major
muscle/tendon units or jeopardizing any neurovascular structures. It is
helpful to be familiar with half-pin systems. If internal fixation is
present, the pin placement can be adjusted to avoid the plates by
customizing the frame. In general, two 5-mm half-pins, medial and
lateral, are required. In larger elbows or in cases in which internal
fixation precludes use of the described sites, a third humeral pin may
be used, usually placed laterally, superior to the spiral groove.
The medial pin is usually placed first through a
two-hole Rancho cube on the undersurface of the upper ring. Both
cortices should be engaged.
The lateral pin is usually placed using a two-hole post and a single-hole Rancho cube (Fig. 8-23). The lateral flare of the humerus is used for placement. The drill guide rests on

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the lateral supracondylar ridge, directed anterior and distally. The
radial nerve, at this level, is anterior to the pin. Humeral fixation
and alignment of the axis of the hinge must be achieved before fixation
of the ulna.

Figure 8-22. Once applied, the frame must freely slide from medial to lateral.
Figure 8-23. The initial fixation is with a lateral half-pin commonly deviated medially at the lateral humeral flare.
One 5-mm and one or two 4-mm pins are used in the ulna.
The more proximal pin (5-mm) provides optimal control of the joint and
is placed form the dorsal surface through the coronoid (Fig. 8-24).
The smaller (4-mm) pins are used more distally in the ulna, again from
the dorsal surface. If the elbow is grossly unstable, it is quite
important to reduce the elbow by placing it in approximately 90 degrees
of flexion when applying the ulnar fixation. Once the joint is reduced
and held in position, the first two proximal ulnar pins can be placed.
Once the first two pins are in place, ranging through flexion and
extension and ensuring reduction of the joint is important. If there is
a tendency for the elbow to subluxate, then alignment has not been
achieved and the bolts must be loosened and reduction achieved.
Figure 8-24. Proximal and distal ulnar pins are used to stabilize the frame to the ulna.
Figure 8-25. Distraction is accomplished by rotating the bolts near the ulnar ring fixation blocks.

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Once the joint has been reduced and all pins applied,
distraction can be applied to the system through the distraction
mechanism. Distraction is achieved by turning the bolts located on the
ulnar ring fixation blocks (Fig. 8-25). Both
sides of the hinge should be distracted an equal amount. Use and extent
of distraction should be done at the discretion of the surgeon.
Postoperative Management
The following general steps are employed
postoperatively, but this plan is individualized according to the
specific needs of the case.
  • The patient is assessed in the recovery room to ensure neurovascular competence.
  • To avoid unwanted joint movement during
    the first 24 hours, the external fixator can be locked by the specific
    mechanism of either device. A Hoffmann II Compact rod is placed between
    the proximal humeral rod and the distal ulnar rod of the DJD II. The
    Compass hinge can also be locked at the distraction if desired.
  • If the procedure requires early motion
    and complete relief of pain, appropriate analgesia should be provided
    to attain this goal. We often employ a brachial plexus catheter for
    this purpose.
  • The patient is encouraged to begin passive range of motion during the first 24 to 48 hours.
  • A careful inspection of the elbow is made
    to assess for swelling and to ensure that the device is not exerting
    pressure on the skin.
  • Proper pin site care is necessary to reduce the risk of pin tract infection.
    TABLE 8-1.
    Post-op management Time period
    Axillary block Recovery room to 48 hours
    CPM Day 1–4: Hospital—then
    Day 4–21: Stiffness
    Day 4–42: Fracture
    Removal of distractor 3 weeks, soft tissue
    6 weeks, fracture
    Flexion and extension splints program 12 weeks
    21 hr/d, 3 weeks
    18 hr/d, 3 weeks
    15 hr/d, 6 weeks
    Long-term splints Maintenance (night)
    3 months
    (longer as needed)
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  • If there is no evidence of infection and
    there has been adequate progress, the patient is dismissed upon
    surgeon’s discretion with active or passive range-of-motion
    instructions.
  • Approximately 3 weeks after the operative
    procedure for stiffness and 6 weeks after treatment of fracture, before
    the fixator is completely removed, the elbow is examined for stability.
    Care is taken not to forcefully manipulate the elbow. If the elbow is
    found to be unstable with the DJD II, the ulnar rod is reattached to
    the ulnar pins. If the elbow is stable, the fixator may be removed as
    well as the ulnar and humeral pins.
An anteroposterior and lateral radiograph is taken to
ensure that the elbow is adequately reduced and stable. The patient is
then treated with flexion and extension splints according to the merits
of the case (Table 8-1).
Illustrative Case
A 28-year-old male sustained a fracture/dislocation with
a type III radial head fracture. The subluxation persisted for 3 weeks
before treatment (Fig. 8-26A). The fracture was managed

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with osteosynthesis (Fig. 8-26B). The construct was protected or “neutralized” by the DJD II (Fig. 8-26C).

Figure 8-26. Complex dislocation with a Mason III radial head fracture dislocation (A).
The fracture is fixed with intrafragment compression osteosynthesis and
the construct is stabilized by the DJD II external fixator (B). The fracture healed with an arc of 20 to 140 degrees of flexion and no pain (C).
Recommended Readings
1. Cobb TK, Morrey BF. Use of distraction arthroplasty in unstable fracture dislocations of the elbow. Clin Orthop 1995;312:201.
2. Kasparayan NG, Hotchkiss RN. Dynamic skeletal fixation in the upper extremity. Hand Clin 1997;13:643.
3. Morrey BF. Distraction arthroplasty: clinical applications. Clin Orthop 1993;293:46.
4. Morrey BF. Post-traumatic contracture of the elbow: operative treatment including distraction arthroplasty. J Bone Joint Surg 1990;72A:601.
5. Morrey BF, Chao EY. Passive motion of the elbow joint. J Bone Joint Surg 1976;58A:501.
6. Morrey BF, Hotchkiss RN. External fixators of the elbow. In: Morrey BF, ed. The elbow and its disorders, 3rd ed. Philadelphia: WB Saunders; 2000.
7. Shiba R, Sorbie C, Siu DW, et al. Geometry of the humeroulnar joint. J Orthop 1988;6:897–906.
8. Tomaino
MM, Sotereanos DG, Plakseychuk A. Technique for ensuring ulnohumeral
reduction during application of the Richards compass elbow hinge. Am J Orthop 1997;26:646.

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