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Valgus Stability of the Elbow: a Definition of Primary and Secondary Constraints

·Clin Orthop Relat Res·1991·571 citations·Shoulder & Elbow
DOI
SummaryAbstract on publisher site →

This cadaveric biomechanical study from Morrey et al. used electromagnetic motion tracking to define the relative contributions of the MCL and radial head to elbow valgus stability during simulated active motion. By serially releasing each structure in varying order across six specimens, the study established a primary-versus-secondary constraint framework that directly guides management of radial head fractures.

Study Snapshot

Design
Cadaveric biomechanical study
Setting: Biomechanics Laboratory, Mayo Clinic
Funding: NIH (AR 26287)
Objective
Whether serial release of the MCL and radial head differentially alters elbow valgus and rotatory kinematics.
Outcome(s)
Three-dimensional abduction-adduction and axial rotation during elbow flexion
Subjects
6 cadaveric elbow specimens (plus 2 preliminary)
  • 3MCL released first, then radial head removed
  • 3Radial head removed first, then MCL released
Inclusion
  • Normal elbow motion and stability on exam
  • Fresh unembalmed specimens
  • Single freeze-thaw cycle only
Exclusion
  • Pre-existing instability or abnormal motion
  • Multiple freeze-thaw cycles
Statistics
Descriptive statisticsMean and standard deviation

Key Findings

  • Radial head removal in an intact elbow produces no measurable change in valgus or axial rotation kinematics. This finding directly supports excision (without prosthetic replacement) for comminuted radial head fractures when the MCL is intact.
  • The anterior bundle of the MCL is the primary valgus stabilizer. Isolated anterior bundle release produced average valgus laxity increases of 3.0°, 3.8°, and 4.8° across the three specimens — even with the radial head still present.
  • The posterior bundle of the MCL contributes minimally to valgus stability. Isolated posterior bundle release produced no measurable change in abduction-adduction rotation.
  • Loss of both stabilizers causes gross instability and elbow subluxation, demonstrated by dislocation at ~120° of flexion in all specimens with both structures absent. When both are gone, the elbow fails.
  • Normal elbow valgus laxity peaks at 10°-20° of flexion with a mean of ~5° (range 3°-7°) under gravity valgus stress. Greatest instability in the combined-release model occurred at 40°-60° of flexion; the elbow was most stable at full extension and full flexion.
  • Simulated muscle activity reduced valgus laxity but could not restore normal stability. After MCL + radial head release, muscle loading restored stability roughly equivalent to an MCL-deficient elbow only. Dynamic stabilizers compensate partially, not fully.
Board PearlThe anterior MCL is the primary valgus stabilizer of the elbow; the radial head is secondary and matters only when the MCL is deficient.

Clinical Relevance

The central clinical question this paper answers is straightforward: does a comminuted radial head fracture require prosthetic replacement? The answer depends entirely on the MCL. With an intact MCL, radial head excision does not alter elbow kinematics — a prosthesis adds nothing.

In contrast, when the MCL is disrupted (think Mason Type 4 fracture-dislocation), the radial head becomes the sole remaining valgus stabilizer. In that setting, excising it without replacement or fixation will produce gross instability.

The practical decision rule: evaluate the MCL before deciding what to do with the radial head. If the MCL is intact, excise a comminuted fragment without implant. If the MCL is compromised, fix or replace the radial head.

The finding that the posterior MCL bundle is nearly irrelevant to valgus stability, while the anterior bundle is decisive, also matters for surgical planning. UCL reconstructions target the anterior bundle for exactly this reason.

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|

Valgus Stability of the Elbow: a Definition of Primary and Secondary Constraints

·Clin Orthop Relat Res·1991·571 citations·Shoulder & Elbow
DOI
SummaryAbstract on publisher site →

This cadaveric biomechanical study from Morrey et al. used electromagnetic motion tracking to define the relative contributions of the MCL and radial head to elbow valgus stability during simulated active motion. By serially releasing each structure in varying order across six specimens, the study established a primary-versus-secondary constraint framework that directly guides management of radial head fractures.

Study Snapshot

Design
Cadaveric biomechanical study
Setting: Biomechanics Laboratory, Mayo Clinic
Funding: NIH (AR 26287)
Objective
Whether serial release of the MCL and radial head differentially alters elbow valgus and rotatory kinematics.
Outcome(s)
Three-dimensional abduction-adduction and axial rotation during elbow flexion
Subjects
6 cadaveric elbow specimens (plus 2 preliminary)
  • 3MCL released first, then radial head removed
  • 3Radial head removed first, then MCL released
Inclusion
  • Normal elbow motion and stability on exam
  • Fresh unembalmed specimens
  • Single freeze-thaw cycle only
Exclusion
  • Pre-existing instability or abnormal motion
  • Multiple freeze-thaw cycles
Statistics
Descriptive statisticsMean and standard deviation

Key Findings

  • Radial head removal in an intact elbow produces no measurable change in valgus or axial rotation kinematics. This finding directly supports excision (without prosthetic replacement) for comminuted radial head fractures when the MCL is intact.
  • The anterior bundle of the MCL is the primary valgus stabilizer. Isolated anterior bundle release produced average valgus laxity increases of 3.0°, 3.8°, and 4.8° across the three specimens — even with the radial head still present.
  • The posterior bundle of the MCL contributes minimally to valgus stability. Isolated posterior bundle release produced no measurable change in abduction-adduction rotation.
  • Loss of both stabilizers causes gross instability and elbow subluxation, demonstrated by dislocation at ~120° of flexion in all specimens with both structures absent. When both are gone, the elbow fails.
  • Normal elbow valgus laxity peaks at 10°-20° of flexion with a mean of ~5° (range 3°-7°) under gravity valgus stress. Greatest instability in the combined-release model occurred at 40°-60° of flexion; the elbow was most stable at full extension and full flexion.
  • Simulated muscle activity reduced valgus laxity but could not restore normal stability. After MCL + radial head release, muscle loading restored stability roughly equivalent to an MCL-deficient elbow only. Dynamic stabilizers compensate partially, not fully.
Board PearlThe anterior MCL is the primary valgus stabilizer of the elbow; the radial head is secondary and matters only when the MCL is deficient.

Clinical Relevance

The central clinical question this paper answers is straightforward: does a comminuted radial head fracture require prosthetic replacement? The answer depends entirely on the MCL. With an intact MCL, radial head excision does not alter elbow kinematics — a prosthesis adds nothing.

In contrast, when the MCL is disrupted (think Mason Type 4 fracture-dislocation), the radial head becomes the sole remaining valgus stabilizer. In that setting, excising it without replacement or fixation will produce gross instability.

The practical decision rule: evaluate the MCL before deciding what to do with the radial head. If the MCL is intact, excise a comminuted fragment without implant. If the MCL is compromised, fix or replace the radial head.

The finding that the posterior MCL bundle is nearly irrelevant to valgus stability, while the anterior bundle is decisive, also matters for surgical planning. UCL reconstructions target the anterior bundle for exactly this reason.

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