Osteotomy Biomechanics

For Medical Students

The Principle

To move the line that load follows, you change the shape of a bone.

Cutting a bone and opening a wedge lengthens that side. Cutting and removing a wedge shortens it. Both tilt what sits above.

A bone is a three dimensional object, so a cut made to change one angle changes others at the same time.

At the Knee

A varus knee is usually corrected by cutting the top of the tibia and opening it on the inner side. 7.69 covers the operation.

That shifts the mechanical axis outwards, so the inner compartment carries less. 3.13.1 covers the axis.

A valgus knee is usually corrected at the bottom of the femur instead. 7.70 covers it.

Either way the load moves to the other compartment, which has to be worth loading. 3.13.8 covers that.

What Changes It

Where the cut is made, which decides what tilts. 3.13.6 covers the joint line.

How much the wedge opens, which decides how far the axis moves. 3.13.10 covers the targets.

Whether the bone heals, which an opened wedge depends on more than a closed one.

The state of the compartment receiving the load.

Clinical Relevance

It is an operation that keeps the patient's own joint, which matters most in younger people. 6.76 covers the young patient.

Recovery is longer than for a joint replacement, because bone has to heal.

The cut also changes the slope of the shin bone and the height of the kneecap. 3.12.12 covers the kneecap.

MRCS Level

The Principle

An osteotomy is a controlled deformity created to cancel an existing one, and the cancellation is only exact in the plane it was planned in.

A single oblique cut changes coronal alignment, sagittal slope and rotation together, because one cut cannot be confined to one plane.

Planning therefore means choosing which unintended changes to accept.

At the Knee

A medial opening wedge high tibial osteotomy shifts the axis laterally, and it also increases posterior tibial slope if the anterior gap exceeds the posterior one.

A lateral closing wedge shortens the limb slightly, heals with bone to bone contact, and tends to reduce slope. 7.69 covers both.

An opening wedge lowers the patella relative to the joint line, because the tubercle moves distally with the tibial shaft. 3.12.12 covers patellar height.

Changing coronal alignment also changes the extensor vector, so the patellofemoral joint is affected whether or not it was the target. 3.12.11 covers that.

A distal femoral osteotomy for valgus has the joint line orientation as its main constraint. 7.70 covers it.

What Changes It

The level and the obliquity of the cut, which set which planes move.

Opening against closing wedge, which changes slope, limb length and healing demands in opposite directions.

Hinge position and integrity, since a fractured hinge loses control of the correction.

Fixation, which decides whether the planned correction is the achieved one.

Bone quality, which decides whether an opened gap heals.

How It Is Measured

Preoperative planning on a standardised standing long leg film, with the hinge and the wedge drawn. 5.16.5 covers it.

Intraoperative assessment with a cable or rod across the limb, which checks the axis directly.

Postoperative long leg radiography, which confirms the coronal correction.

Lateral radiography for slope and patellar height, which are the changes most often unmeasured afterwards.

Contact pressure change is inferred from the geometry rather than measured. 3.5.10 covers it.

Clinical Relevance

Plan the hinge and the wedge on a standardised film rather than improvising the correction in theatre. 5.16.5 covers the planning.

Measure slope and patellar height before and after, because both change and both matter. 3.12.12 covers patellar height.

Protect the hinge, since a hinge fracture converts a planned correction into an unplanned one.

Expect an opening wedge to demand more of bone healing than a closing wedge does.

FRCS Level

At the Knee

The mechanical objective is to reduce the adduction moment's effect on the medial compartment, and the operation achieves that by shortening the moment arm. 3.13.7 covers the moment.

Because only the arm changes, the reduction in medial load is smaller than the change in degrees implies, and that is consistent with the clinical results. 3.5.10 covers the contact mechanics.

The sagittal consequence is the one most often missed. Slope change alters cruciate loading, and an increase loads an anterior cruciate graft. 3.7.20 covers that.

Deliberate slope change is now a procedure in its own right in the revision cruciate knee. 7.74 covers combined procedures.

Joint line obliquity sets the ceiling on single level correction, which is why large deformities go to two levels. 3.13.6 covers it.

What Changes It

The proportion of deformity that is intra-articular, which the osteotomy cannot address. 3.13.6 covers it.

Wedge geometry in the sagittal plane, which decides the slope change.

The compartment receiving the load, whose cartilage status limits the benefit. 5.11 covers the imaging.

Ligament status, because an osteotomy in a lax knee changes the loading of the restraints too. 3.7.20 covers the cruciate interaction.

Patellofemoral status, which an opening wedge worsens by lowering the patella. 3.12.12 covers it.

How It Is Measured

Digital planning with the hinge, the wedge and the predicted obliquity, which makes the trade-offs explicit. 5.16.5 covers it.

Lateral radiography for slope, measured by a stated method, since the methods differ.

Patellar height by a named ratio before and after. 3.12.12 covers the ratios.

Gait analysis for the adduction moment before and after, which would demonstrate the mechanical effect and is rarely done. 3.16.7 covers it.

Published series report radiographic correction and survivorship rather than the mechanical quantity the operation targets.

Clinical Relevance

Plan the sagittal consequence as deliberately as the coronal correction, because slope changes cruciate load. 3.7.20 covers it.

Measure patellar height with a named ratio before and after an opening wedge.

Assess the receiving compartment, since the benefit is limited by its cartilage. 5.11 covers the imaging.

Expect the medial load reduction to be smaller than the degrees suggest, and say so when counselling.

Report radiographic correction while stating that the targeted mechanical quantity was not measured.

Fellowship Level

At the Knee

At fellowship level the osteotomy is a three plane operation used for several different purposes, and the purpose decides which plane leads.

For medial compartment disease the coronal plane leads and the sagittal plane is a constraint. For a revision cruciate knee with a steep slope the sagittal plane leads and the coronal plane is the constraint. 3.7.20 covers the cruciate case.

Combining an osteotomy with a ligament reconstruction or a cartilage procedure changes the sequencing and the fixation, and the osteotomy usually goes first or at the same sitting. 7.73 and 7.74 cover the combinations.

Revision osteotomy is a different problem, because the deformity now includes a surgical component with its own hinge and hardware. 7.75 covers it.

Conventional teaching has moved towards double level correction for large deformities, and towards planning slope rather than accepting it.

What Changes It

Which plane the indication leads with, which decides the wedge geometry.

Concurrent procedures, which change sequencing, fixation and rehabilitation. 8.42 covers combined procedures.

Previous surgery, including hardware, a healed hinge and an altered joint line. 7.75 covers revision.

Patellofemoral status, which an opening wedge can convert from asymptomatic to symptomatic. 3.12.12 covers it.

How It Is Measured

Three dimensional planning from computed tomography, which handles multiplanar corrections that a radiograph cannot. 3.16.5 covers it.

Patient specific guides printed from that plan, which transfer it to within a degree or two.

Intraoperative navigation, which measures the achieved correction directly and adds registration error. 3.16.3 covers it.

Postoperative three plane assessment, which is the only check on the unintended planes.

No series reports the adduction moment, the slope and the patellar height together before and after, which is the measurement set the operation actually changes.

Clinical Relevance

State which plane leads for the indication, and treat the others as constraints to be planned rather than accepted.

Plan multiplanar corrections in three dimensions, since a radiograph cannot express them. 5.16.5 covers the planning.

Sequence a combined procedure deliberately, with the osteotomy's fixation and healing demands set against the other procedure's rehabilitation. 8.42 covers it.

Treat a failed previous correction as a new deformity with a surgical component rather than as the original one. 7.75 covers revision.

Weigh the patient's age, physiology, comorbidities, knee demand, psychological state and their own ideas, concerns and expectations, because this is a long recovery for a change in a surrogate.