3D Orthognathic Surgery: Enhancing Accuracy through CT and STL Image Analysis
- Dr. Park
- Oct 26, 2024
- 4 min read
Orthognathic surgery, commonly known as jaw corrective surgery, is a highly complex procedure that aims to correct the position and shape of facial bones for both aesthetic and functional improvements.
Successful surgery hinges on three essential factors:
appropriate surgical planning,
precise execution according to the plan,
and ensuring long-term stability.
Accurate execution, in particular, plays a crucial role in increasing patient satisfaction and reducing the possibility of complications.
In the past, surgical planning relied on two-dimensional X-ray images, which increased the likelihood of unpredictable variables during surgery. This inevitably compromised the accuracy of surgical outcomes. However, with advancements in medical technology, the advent of 3D technology in orthognathic surgery has revolutionized the precision and safety of the procedure.
Through extensive experience and continuous research, I have established my own expertise in enhancing the accuracy of 3D orthognathic surgery. In this posting, I'd like to share one of the core aspects of my approach.
What is 3D Orthognathic Surgery?
The key processes involved in 3D orthognathic surgery are as follows:
Three-dimensional CT scan: This provides a comprehensive 3D analysis of the patient's facial bones, enabling accurate diagnosis.
Virtual surgical planning: Utilizing computer software, the surgeon simulates jawbone movement, osteotomy locations, and other parameters to formulate an optimal surgical plan.
Customized fabrication: Patient-specific surgical tools, guides, and metal plates are fabricated using 3D printing or CAM technology.
Consequently, 3D orthognathic surgery offers the following advantages:
Minimizes surgical errors and enhances predictability through 3D analysis, facilitating precise execution according to the plan.
Reduces the risk of nerve damage and other complications.
Enhances patient satisfaction by providing personalized surgical solutions.
However, these advantages are contingent on the surgery proceeding exactly as planned. Therefore, a separate verification process is necessary to confirm the accuracy of the execution.
Is Pre- and Post-operative CT Comparison Sufficient for 3D Orthognathic Surgery?
The answer is no.
While CT scans allow for three-dimensional analysis and diagnosis of facial bones, virtual surgical planning is performed using STL (STereoLithography/Standard Tessellation Language) images, not CT images.
Therefore, to accurately verify the surgical outcome, comparison must be done using STL images.
Nevertheless, pre- and post-operative CT comparison still holds significance.
What are the Differences between CT and STL Comparisons in 3D Orthognathic Surgery?
CT images can be used to assess the following parameters to confirm surgical accuracy:
Jawbone positional changes before and after surgery: Evaluate the degree of deviation of the chin from the skeletal midline, the difference in the width of the left and right jawbones, and the improvement in asymmetry.
Osteotomy line verification: Confirm if osteotomies were performed at the planned locations.
Implant positioning: Verify the accurate placement of implants and their relationship with surrounding tissues.
However, CT images have limitations in quantifying changes in specific areas of interest. Orthognathic surgery plans involve precise numerical values for maxillary and mandibular movements. CT scans struggle to verify if the surgery has achieved these planned movements accurately.
The Need for STL (STereoLithography/Standard Tessellation Language) Comparison
To overcome these limitations, STL comparison is essential. STL files represent the surface of a 3D model as a triangular mesh, which may not perfectly match the actual bone morphology. Therefore, STL analysis should be used as a reference and interpreted in conjunction with other information, such as CT images.
By utilizing both CT and STL images, surgeons can comprehensively evaluate and improve the accuracy of the surgery.
Assessing 3D Orthognathic Surgery Accuracy: A Case Study
Let's examine a real patient case:
Surgical Plan:
Maxillary osteotomy: Maxillary central incisor moved 0.7mm to the right, 6.5mm upward, and 0.5mm forward.
Mandibular osteotomy: Tooth #16 moved 4.01mm upward, tooth #26 moved 4.08mm upward.
A point: Moved 1.92mm posteriorly.
Jaw counter-rotation: Pogonion moved 4.5mm anteriorly.
Contouring surgery: Genioplasty, mandibular angle reduction, and zygomatic reduction.
CT Image Comparison:
Pre and postoperative 2week CT images showing alignment with the skeletal midline
As demonstrated in the CT images, we can confirm the following:
Jawbone positional changes before and after surgery: The chin is aligned with the midline, and the width discrepancy between the left and right jawbones is addressed.
Osteotomy line verification: Osteotomies were accurately performed at the planned locations.
Based on these observations, we can infer that the surgery was executed reasonably close to the plan.
However, our surgical plan was designed with specific numerical values.
CT images struggle to quantify whether the bones and teeth have moved according to these planned values. This is where STL comparison becomes crucial.
STL Image Comparison:
Plan STL (virtual surgical plan) and Post STL (postoperative STL image)
[Video of 3D jaw surgery error analysis]
Surgical Error Analysis:
The STL image displays point numbers, allowing us to track the three-dimensional positional changes of each point after surgery. The displacement values(이동량) represent the surgical errors.
The average absolute error for all measurements is 0.27mm.
Analyzing the tooth region separately reveals an average absolute error of 0.22mm.
Notably, the error for the maxillary teeth is only 0.06mm, indicating that most of the error originates from the mandible.
This discrepancy is likely due to insufficient closure of the mandible, possibly caused by incomplete seating of the lower teeth in the wafer. This analysis prompts further investigation into the reasons for the incomplete seating.
Condyle Position Comparison:
Pre and postoperative condyle position comparison
By comparing the pre- and post-operative condyle positions, we can assess if the insufficient closure is related to the post-operative condyle position.
In this case, there is no significant difference, ruling out condylar position as the cause.
We anticipate that the mandible will gradually settle into the planned position as muscle strength recovers, and therefore no specific intervention is required.
Conclusion:
3D orthognathic surgery, coupled with CT and STL image analysis, provides an objective method for evaluating and enhancing surgical accuracy. 3D technology proves invaluable throughout the entire surgical process, from planning to outcome analysis. With continued dedication from medical professionals and ongoing technological advancements, we can anticipate even more precise and safe surgical procedures in the future.
By Dr. Jong-Chul Park, Oral and Maxillofacial Surgeon
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