Sagittal Plane Assessment in Deformity Correction Planning: The Sagittal Joint Line Angle

Abstract

Aim: Evaluate the validity of a recent approach to calculate the knee flexion or extension contracture contributing to the overall sagittal deformity using the sagittal mechanical axis angle (SMAA) for the overall alignment assessment and sagittal joint line angle (SJLA) for soft tissue contribution. The methods of evaluating these angles and their clinical applications are discussed.

Materials and methods: In total, 107 normal limbs met the criteria and were divided into two groups: skeletally mature and immature. Sagittal alignment was evaluated using the Bone Ninja iPad application, and the posterior distal femoral angle (PDFA), posterior proximal tibial angle (PPTA), SMAA and SJLA were recorded.

Results: In skeletally immature patients, mean SJLA was 13.46° [standard deviation (SD), 4.55°], and in mature patients, it was 16.91° (SD, 2.948°). The PDFA and PPTA were consistent with previously published measurements.

Conclusion: The SJLA method is a practical way to quantify the soft tissue contribution and degree of contracture. It can also be used for monitoring deterioration or improvement of knee range of motion during lengthening or physical therapy.

Clinical significance: All patients in this study presented to our clinic with symptoms on the contralateral side. This, in addition to the retrospective nature, was a limitation in our study.We recommend a validity study to compare our SJLA method to the classic anterior cortical line angle (ACL) method in addition to an inter-observer and intra-observer reliability study for the SJLA. We also recommend a study on completely normal asymptomatic subjects to better standardise the angle measurements in skeletally immature patients at different ages.

How to cite this article: Abalkhail TB, McClure PK. Sagittal Plane Assessment in Deformity Correction Planning: The Sagittal Joint Line Angle. Strategies Trauma Limb Reconstr 2022;17(3):159-164.

Keywords: Deformity correction; Deformity planning; Knee; Range of motion; Sagittal mechanical axis; Soft tissue contractures.

Figs 1A to C

(A and B): A consistent ACL measurement can be difficult to define. Even though each panel shows the same deformity, the ACL measurement may vary depending on how one draws each anterior cortical line. This is particularly evident in metabolic bone disease. (C) SMAA uses anatomic landmarks so that it can be drawn consistently. © 2021, Rubin Institute for Advanced Orthopedics, Sinai Hospital of Baltimore. Used with permission

Fig. 2

Posterior distal femoral angle is the angle between the femur mechanical axis and distal femur joint orientation line. Posterior proximal tibial angle is the angle between the tibia mechanical axis and proximal tibia joint orientation line. Sagittal mechanical axes angle is the angle between the femur mechanical axis and tibia mechanical axis. Sagittal joint line angle is the angle between the distal femur joint orientation line and proximal tibia joint orientation line. © 2021, Rubin Institute for Advanced Orthopedics, Sinai Hospital of Baltimore. Used with permission

Fig. 3

A sagittal joint line angle less than 16° indicates flexion deformity while an SJLA greater than 16° indicates hyperextension deformity. © 2021, Rubin Institute for Advanced Orthopedics, Sinai Hospital of Baltimore. Used with permission

Figs 4A to C

Example of sagittal plane bony and soft tissue deformities. The measurement of the SMAA shows that there is a flexion deformity of 26° that is caused by a distal femoral deformity of 20° flexion with an additional soft tissue contribution. The tibia is normal [PPTA = 81° and anterior distal tibial angle (ADTA) = 80°], and the patient does not have any ankle complaints. © 2021, Rubin Institute for Advanced Orthopedics, Sinai Hospital of Baltimore. Used with permission

Figs 5A and B

Posterior distal femoral angle among (A) immature and (B) mature patients

Figs 6A and B

Posterior proximal tibial angle among (A) immature and (B) mature patients

Fig. 7

Distribution of the adjusted SJLA among all patients. (X axis: adjusted SJLA, Y axis: frequency)

Fig. 8

Adjusted SJLA mean comparison between mature and immature patients