Modern spinal deformity surgery can be planned in much greater detail than was possible in the past. Rather than evaluating X-rays and deciding on correction only after surgery has begun, surgeons can now use computer-based surgical simulation to evaluate spinal alignment, compare different correction strategies, and establish specific alignment goals before entering the operating room.
For selected complex spinal procedures, I use UNiD™ patient-specific planning technology to help simulate the operation and create rods manufactured to match the planned spinal correction.
The goal is not simply to make the spine look straighter on an X-ray. The goal is to create a balanced spinal alignment appropriate for the individual patient while avoiding both inadequate correction and excessive correction.
What Is Spine Surgery Simulation?
Spine surgery simulation uses a patient’s pre-operative imaging to create a detailed plan for reconstructive spinal surgery.
Measurements can be made of the relationship between the:
- Spine
- Pelvis
- Hips
- Head
- Overall standing posture
The planned operation can then be modeled before surgery.
Different surgical strategies can be compared to determine how changes at individual spinal levels may affect the patient’s overall alignment.
This is particularly valuable in adult spinal deformity surgery, where correcting one portion of the spine can influence the position of the rest of the body.
What Is UNiD?
UNiD™ patient-specific rod technology is part of Medtronic’s spinal planning platform.
The system combines pre-operative imaging, computer-assisted planning, surgical simulation, and patient-specific spinal rods.
A surgical plan can be developed using the patient’s alignment measurements and the surgeon’s intended operative strategy. Once the surgeon approves the final plan, rods can be manufactured to correspond to the planned spinal alignment.
The technology is most commonly used for complex spinal reconstruction and deformity surgery.
Step 1: Measuring the Patient’s Spinal Alignment
Planning begins with appropriate standing spinal imaging.
Rather than looking only at the portion of the spine that hurts, complex deformity evaluation may examine the relationship between the entire spine and pelvis.
Measurements may include:
- Pelvic incidence
- Pelvic tilt
- Lumbar lordosis
- Thoracic kyphosis
- Sagittal vertical axis
- Segmental spinal angles
- Regional spinal alignment
- Overall spinal balance
These measurements help identify where the deformity originates and how the patient is compensating for it.
Step 2: Understanding Compensation
The human body naturally attempts to keep the head centered over the pelvis and the eyes facing forward.
When spinal deformity develops, patients may compensate by:
- Tilting the pelvis backward
- Bending the knees
- Extending unaffected portions of the spine
- Rotating the pelvis
- Changing the position of the hips
A patient can therefore appear reasonably balanced while actually using substantial muscular and skeletal compensation.
Computer-assisted planning helps evaluate these relationships before surgery.
Step 3: Simulating the Operation
Once the patient’s alignment has been measured, different surgical strategies can be modeled.
The simulation may evaluate the effects of:
- Interbody fusion
- Segmental correction
- Osteotomies
- Vertebral reconstruction
- Changes in lumbar lordosis
- Changes in thoracic kyphosis
- Correction at specific spinal levels
The surgeon can compare potential strategies before selecting the final operative plan.
Medtronic’s current UNiD/GAiTEWAY workflow incorporates computerized planning and predictive modeling intended to assist surgeons in evaluating alignment and potential postoperative compensation.
Planning Where the Correction Should Occur
Achieving an appropriate total amount of spinal correction is only part of deformity surgery.
Where that correction occurs is also important.
For example, lumbar lordosis is not distributed equally throughout the lumbar spine. The lower lumbar segments normally contribute a substantial portion of the overall lumbar curve.
Computer-based planning allows the surgeon to think about both:
- The total correction
- The distribution of that correction
This can be particularly useful when planning multilevel interbody fusion, osteotomies, or revision deformity surgery.
Avoiding Under-Correction
If inadequate correction is achieved, a patient may remain pitched forward after surgery.
Persistent imbalance can contribute to:
- Difficulty standing upright
- Muscle fatigue
- Back pain
- Continued compensatory posture
- Reduced walking tolerance
Pre-operative simulation allows the surgeon to establish a specific correction goal rather than relying entirely on intraoperative estimation.
Avoiding Over-Correction
More correction is not always better.
The ideal alignment of a 40-year-old patient is not necessarily the ideal alignment of a 75-year-old patient.
Over-correcting the spine can place substantial forces on the remaining mobile segments and the junction between the fused and unfused spine.
For this reason, modern deformity planning considers factors such as:
- Age
- Existing spinal alignment
- Pelvic anatomy
- Flexibility
- Bone quality
- Location of the fusion
- Overall surgical goals
The objective is appropriate correction rather than maximal correction.
Step 4: Creating Patient-Specific Rods
Once the surgical plan is finalized, patient-specific spinal rods can be manufactured according to the planned alignment.
Traditional spinal rods are usually contoured by the surgeon during the operation.
Patient-specific rods are different.
Their shape is established from the pre-operative surgical plan and manufactured before surgery. The rods therefore provide a physical representation of the alignment that was planned before the operation.
Why Does Rod Shape Matter?
The rods connecting spinal screws help determine the final shape of the reconstructed spine.
In a long spinal fusion, relatively small differences in rod contour can produce meaningful differences in:
- Lumbar lordosis
- Thoracic kyphosis
- Segmental alignment
- Overall sagittal balance
Reproducing a complex planned contour manually in the operating room can be challenging.
Patient-specific rods can help translate the pre-operative alignment plan into the actual surgical construct.
The Rod Is a Guide, Not a Substitute for Surgical Judgment
Patient-specific rods do not make the operation automatic.
The surgeon still determines:
- Which levels should be treated
- Where decompression is necessary
- Whether interbody fusion is needed
- Whether an osteotomy is necessary
- How much correction is safe
- Whether the surgical plan needs to change during the operation
Patient anatomy, bone quality, nerve monitoring, surgical findings, and the behavior of the spine during correction may require modification of the original plan.
The technology is therefore a planning and execution tool rather than a replacement for surgical judgment.
When Might I Use Spine Surgery Simulation?
Patient-specific planning is most useful when alignment plays a major role in the operation.
Examples may include:
- Adult degenerative scoliosis
- Flat-back syndrome
- Sagittal imbalance
- Kyphosis
- Revision spinal deformity surgery
- Adjacent segment deformity
- Pseudarthrosis with loss of alignment
- Long thoracolumbar fusion
- Complex spinal reconstruction
It is generally unnecessary for routine procedures such as a one-level lumbar decompression or straightforward microdiscectomy.
Adult Spinal Deformity
Adult spinal deformity is one of the most important applications of surgical simulation.
Patients may have combinations of:
- Scoliosis
- Loss of lumbar lordosis
- Forward spinal imbalance
- Pelvic compensation
- Thoracic deformity
- Previous fusion
- Adjacent segment degeneration
Correcting these problems requires understanding the entire spinal column rather than treating a single abnormal disc or vertebra.
Revision Spine Surgery
Computer-assisted planning can be particularly helpful in revision surgery.
A previous fusion may have healed in an alignment that no longer allows the patient to stand comfortably upright.
Other patients develop:
- Adjacent segment disease
- Pseudarthrosis
- Hardware failure
- Progressive deformity
- Flat-back syndrome
Planning software can help determine where the existing deformity originates and model how a revision operation might restore more appropriate alignment.
Planning Osteotomies
Some rigid spinal deformities cannot be corrected simply by positioning the patient or placing interbody cages.
A controlled cut through bone, called an osteotomy, may be required.
Simulation can help estimate:
- How much correction is needed
- Where that correction should occur
- Whether smaller corrections at several levels may be sufficient
- Whether a larger osteotomy is necessary
This can help create a more deliberate operative strategy before surgery begins.
Planning Interbody Correction
Interbody cages placed between vertebrae can also change spinal alignment.
Computer simulation can help plan:
- Which levels should receive interbody fusion
- How much correction should occur at each level
- How the combined changes affect overall alignment
This is particularly useful when several lumbar levels need to be reconstructed.
Relationship to Navigation and Robotics
Pre-operative planning, patient-specific rods, navigation, and robotics are different technologies, but they can complement one another.
A modern workflow may involve:
Planning
Determine the desired spinal alignment before surgery.
Navigation or robotics
Assist with accurate placement of screws and other implants.
Patient-specific rods
Help reproduce the planned spinal contour.
Intraoperative imaging
Confirm implant position and, in selected settings, evaluate alignment.
Current Medtronic platforms are designed to integrate planning, patient-specific rods, navigation and robotic technologies within a broader surgical workflow.
Does Simulation Predict Exactly What Will Happen?
No.
The human spine is not a rigid computer model.
Several factors can affect the final result, including:
- Spinal flexibility
- Positioning during surgery
- Bone quality
- Muscle forces
- Hip and pelvic compensation
- Surgical releases
- Osteotomies
- Interbody cage position
- Individual healing
Simulation provides additional information for planning, but it cannot perfectly predict the result of every operation.
Does Using Custom Rods Make Spine Surgery Safer?
Patient-specific planning may help surgeons execute a predefined alignment strategy more consistently, but complex spinal reconstruction still carries significant risks.
These can include:
- Neurologic injury
- Infection
- Bleeding
- Dural tear
- Nonunion
- Hardware failure
- Junctional problems
- Need for additional surgery
Technology can improve the information available to the surgical team, but it does not eliminate the underlying risks of complex spine surgery.
Why Use This Technology?
For selected patients, I believe the most important advantage is the ability to make complex alignment decisions before the operation rather than relying entirely on decisions made in the operating room.
The process allows the surgical team to:
- Quantify the deformity
- Establish specific alignment goals
- Compare different correction strategies
- Plan where correction should occur
- Create rods corresponding to the planned alignment
- Use the plan as a reference during surgery
This is particularly valuable in large spinal reconstructions where small differences in alignment can have significant consequences.
A More Personalized Approach to Spinal Reconstruction
Two patients with similar X-rays may not require exactly the same spinal correction.
Age, pelvic anatomy, flexibility, previous surgery, bone quality, symptoms, and overall spinal alignment all influence the surgical plan.
Patient-specific planning allows spinal reconstruction to move away from a single standardized alignment target and toward a more individualized strategy for each patient.
Is UNiD Planning Appropriate for Every Patient?
No.
Most spine operations do not require patient-specific rods or advanced deformity simulation.
The technology is most useful when:
- The operation involves a long fusion
- Significant deformity is being corrected
- Sagittal or coronal alignment is a major concern
- A previous fusion requires reconstruction
- Osteotomies are being considered
- Precise distribution of correction is important
The decision to use advanced planning technology should depend on whether it meaningfully contributes to the operation.
Outlook
Spinal deformity surgery increasingly combines traditional surgical principles with advanced imaging, computer simulation, navigation, robotics, and patient-specific implants.
UNiD patient-specific planning allows a proposed spinal reconstruction to be evaluated before surgery and can translate the approved alignment plan into custom-manufactured spinal rods.
For complex adult deformity surgery, these tools can provide another layer of planning between identifying the patient’s deformity and performing the actual reconstruction.
The technology does not replace the surgeon. Its value is in giving the surgeon better information, a more detailed plan, and a patient-specific target to guide a complex operation.
Related Topics
- Adult Spinal Deformity
- Adult Degenerative Scoliosis
- Sagittal Imbalance
- Flat-Back Syndrome
- Spinal Osteotomy
- Pedicle Subtraction Osteotomy
- Revision Spine Surgery
- Thoracolumbar Spinal Fusion
- Robotic Spine Surgery
- Spinal Navigation
- O-Arm Intraoperative Imaging