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

INTRODUCTION

Preoperative planning is essential before any successful orthopaedic surgical intervention. Failing to plan is planning to fail.^1,2 Many studies have demonstrated not just the importance but also the methods of preoperative planning in several fields, including (but not limited to) fracture, arthroplasty and deformity surgeries.^2–6 The significance of this step in surgical treatment is also supported by the development of several computer software programmes for preoperative templating and planning.^7,8

It is crucial to evaluate sagittal plane deformities and correct them when necessary, as they may cause harm to knee joint dynamics.^9–12 Deformities in this plane are more likely to go unnoticed and uncorrected, largely a result of two reasons. First, they tend to be more tolerable to patients because they occur within the plane of motion. Second, surgeons who do not routinely deal with deformity cases may only look at the frontal plane view. It may be true that sagittal plane deformities are less common than their frontal plane counterparts, but due diligence should lead the surgeon to look at both planes. It is worth noting that the remodelling potential in the sagittal plane is relatively better, particularly in the paediatric population.¹³ Although it has been suggested that sagittal deformity is both more well-tolerated and remodels more readily, this has not been strictly evaluated or proven. Some long-term follow-up studies demonstrate significant risk of arthritis secondary to sagittal deformity.^11,12,14 It is possible that sagittal deformity is quite significant but remains silent until early degenerative disease occurs.

Standard joint-orientation angles of a well-aligned limb in the frontal plane have been well-studied and are widely accepted.^15,16 However, little published data exist about the standard angles in a sagittal plane.¹⁷ Historically, the ACL was used in conjunction with PDFA and PPTA as a proxy to ascertain the knee soft tissue contribution—either flexion or extension contracture—to the overall sagittal malalignment. We have found inconsistency when drawing the lines in our experience with this method, making it unreliable (Fig. 1). We evaluate the validity of a recent approach by McClure et al.¹⁸ to calculate the knee flexion or extension contracture contributing to the overall sagittal deformity, using what is called SMAA for the overall alignment assessment and SJLA for soft tissue contribution.

The SJLA is formed between the sagittal distal femur and proximal tibia joint lines of a maximally extended knee. This is similar to the frontal plane’s joint line convergence angle (JLCA), which occurs between the frontal plane’s distal femur and proximal tibia joint lines and is used to evaluate the laxity in the knee collateral ligaments.^15,16 Normal PDFA range is 79–87°, and normal PPTA range is 77–84°; we hypothesize that a normal SJLA would range between 9° and 24°, with 16° being the median. In this study, we test this hypothesis, assess if 16° is the mean SJLA and demonstrate how this angle can be utilised in sagittal plane evaluation and deformity correction planning.

MATERIALS AND METHODS

Institutional review board approval was obtained prior to conducting this observational, retrospective study occurring at a single centre. Clinical data were reviewed from >300 patients’ electronic medical records and long-leg lateral X-rays from 2012 to 2019. The study included either limbs from patients with no lower limb abnormalities or otherwise the contralateral normal side of patients who presented with various unilateral pathologies. The exclusion criteria were the presence of any deformity in any plane, previous surgeries, or improperly rotated X-rays that precluded accurate angle measurements. If none of these exclusions were present, and if the condyles superimposed perfectly, the X-rays were granted inclusion. In total, 107 limb X-rays met our criteria.

X-rays were obtained from our picture archiving and communication system, converted to JPEG, and evaluated using the Bone Ninja application (International Center for Limb Lengthening, Rubin Institute for Advanced Orthopedics, Sinai Hospital of Baltimore, Baltimore, MD) for iPad (Apple, Cupertino, CA). Statistical analysis was conducted with SPSS v17.0 (IBM, Armonk, NY). A p-value <0.05 was considered statistically significant.

Subjects were divided into two groups: skeletally mature and immature, with group assignment based on physis closure. We evaluated the long leg lateral X-rays, including measuring the SMAA, PDFA, PPTA, and SJLA. All angles were measured by the senior author PKM. Some X-rays were taken with slight knee flexion and not the ideal maximum knee extension, or had a normal variant of slight knee hyperextension. For these, we would note an adjusted SJLA which we determined by subtracting the SMAA from the SJLA. For example, a patient with an SJLA of 16° and an SMAA of 2° (2° of hyperextension) will have an adjusted SJLA of 14°.

RESULTS

In total, 107 limbs were reviewed, 50 of which were skeletally immature (47.7%) and 57 of which were mature (53.3%). Overall mean PDFA was 83.07° (SD, 3.75°). In skeletally immature patients, mean PDFA was 84.32° (SD, 4.41°), whereas, in mature patients, it was 81.96° (SD, 2.639°) (Fig. 5). Overall mean PPTA was 81.4° (SD, 3.189°). In skeletally immature patients, mean PPTA was 81.44° (SD, 3.955°), and in mature patients, it was 81.37° (SD, 2.358°) (Fig. 6). The mean-adjusted SJLA in all 107 patients was 15.3° (SD, 4.142°) (Fig. 7). In skeletally immature patients, mean SJLA was 13.46° (SD, 4.55°), and in mature patients, it was 16.91° (SD, 2.948°) (Fig. 8).

DISCUSSION

In our study, the PDFA and PPTA overall means were 83.07° and 81.4°, respectively. These findings are consistent with previously published measurements.¹⁵ These standard measurements were based on a study on completely asymptomatic adult patients. In our study, the subjects were a combination of skeletally mature and immature patients with pathology on the contralateral side.

Solomin et al.¹⁷ studied 23 individuals with no deformities in any plane and, based on computed tomography (CT), found that the lower limb mechanical axis in the sagittal plane intersects and divides the joint line of the distal femur to a 43.8 ± 7.9% segment anteriorly and 56.2 ± 7.9% posteriorly (i.e., 2/5 anteriorly and 3/5 posteriorly). This differs from the research by Standard et al.¹⁵ wherein the anatomical axis of the distal femur divides the joint line to 1/3 anteriorly and 2/3 posteriorly. They found that the mechanical axis line extends to divide the proximal joint line of the proximal tibia and divides it into a 23.3 ± 8.8% segment anteriorly and 76.7 ± 8.8% posteriorly (1/5 anteriorly and 4/5 posteriorly). Standard et al.¹⁵ also recorded the PDFA as 81.1 ± 3.95° and the PPTA as 81.6 ± 2.8°.

When focusing on the skeletally immature patients, the mean PDFA was 84.32° and the mean PPTA was 81.44°, indicating that standard measurements for skeletally immature patients need to be customised. The mean SJLA in skeletally immature patients was 13.46° (SD, 4.55°). This could be attributed to the differences in angles measured between different ages in the skeletally immature group. We experienced some difficulty drawing the lines in the immature group, which may have created inconsistency when measuring the PDFA, PPTA and SJLA. We recommend an inter-observer and intra-observer reliability study to confirm this.

The ACL method is commonly used to quantify the overall alignment in the sagittal plane.¹⁶ In our opinion, this method is unreliable because there are different ways a surgeon might draw the ACL, and moreover, it can only be used in a portion of sagittal deformities (Fig. 1). Additionally, the ACL does not take into consideration the proximal femur and distal tibia.¹⁵ By moving the points that define a line further apart (femoral head and 1/3 instead of best fit of anterior rule of distal 1/3), the variability in line selection decreases.

Vilenskiy et al.¹⁹ reviewed CTs in 23 adult patients with nondeformed femurs and demonstrated that the angle between the proximal femoral anatomical and mechanical axes on the sagittal plane is 10.2° ± 2.4°, and the angle between the femoral neck and the mechanical axis of femur in the sagittal plane is 16.0° ± 7.6°. This is a practical way of drawing the proximal mechanical axis of a deformed femur in the sagittal plane when planning correction. This study focused on femur planning rather than the overall sagittal plane assessment, particularly the joint contribution. The absence of a torsional deformity on the femur was a prerequisite in this study.

Jud et al.²⁰ found that a rotational osteotomy of the femur closer to the knee, as opposed to the hip, exerts more overall sagittal axis deviation. It was noted in our study that the anteversion and retroversion in the femur could affect the sagittal alignment. This observation needs to be studied to determine the effect of the femur neck version on the sagittal alignment, if any.

CONCLUSION

In contrast with the frontal plane, understanding the soft tissue contribution to the sagittal mechanical axis is more commonly relevant, and careful evaluation is essential when analysing and correcting a sagittal plane deformity.

The SJLA method we describe is useful when analysing sagittal plane deformities. Sagittal joint line angle analysis is a practical way to quantify the soft tissue contribution and degree of contracture. The SJLA can also be used for monitoring improvement or deterioration of knee range of motion during lengthening or physical therapy.

ORCID

Philip K McClure https://orcid.org/0000-0003-4379-7622

REFERENCES

1. Graves ML. The value of preoperative planning. J Orthop Trauma 2013;27(Suppl 1): S30–S34. DOI: 10.1097/BOT.0b013e3182a52626.

2. Atesok K, Galos D, Jazrawi LM, et al. Preoperative planning in orthopaedic surgery. Current practice and evolving applications. Bull Hosp Jt Dis (2013) 2015;73(4):257–268. PMID: 26630469.

3. Tanzer M, Makhdom AM. Preoperative planning in primary total knee arthroplasty. J Am Acad Orthop Surg 2016;24(4):220–230. DOI: 10.5435/JAAOS-D-14-00332.

4. Okike K. Preoperative planning in fracture surgery: The end of colored pens and tracing paper?: Commentary on an article by Yanxi Chen, MD, PhD, et al.: “Computer-assisted virtual surgical technology versus three-dimensional printing technology in preoperative planning for displaced three and four-part fractures of the proximal end of the humerus.” J Bone Joint Surg Am 2018;100:e146.

5. Eggli S, Pisan M, Müller ME. The value of preoperative planning for total hip arthroplasty. J Bone Joint Surg Br 1998;80(3):382–390. DOI: 10.1302/0301-620x.80b3.7764.

6. Paley D, Herzenberg JE, Tetsworth K, et al. Deformity planning for frontal and sagittal plane corrective osteotomies. Orthop Clin North Am 1994;25(3):425–465. PMID: 8028886.

7. Whitaker AT, Gesheff MG, Jauregui JJ, et al. Comparison of PACS and Bone Ninja mobile application for assessment of lower extremity limb length discrepancy and alignment. J Child Orthop 2016;10(5):439–443. DOI: 10.1007/s11832-016-0761-5.

8. Westacott DJ, McArthur J, King RJ, et al. Assessment of cup orientation in hip resurfacing: A comparison of TraumaCad and computed tomography. J Orthop Surg Res 2013;8:8. DOI: 10.1186/1749-799X-8-8.

9. Maderbacher G, Baier C, Springorum HR, et al. Lower limb anatomy and alignment affect natural tibiofemoral knee kinematics: A cadaveric investigation. J Arthroplasty 2016;31(9):2038–2042. DOI: 10.1016/j.arth.2016.02.049.

10. Lindahl O, Movin A, Ringqvist I. Knee extension. Measurement of the isometric force in different positions of the knee-joint. Acta Orthop Scand 1969;40(1):79–85. DOI: 10.3109/17453676908989487.

11. Palmu SA, Lohman M, Paukku RT, et al. Childhood femoral fracture can lead to premature knee-joint arthritis. 21-year follow-up results: A retrospective study. Acta Orthop 2013;84(1):71–75. DOI: 10.3109/17453674.2013.765621.

12. Agneskirchner JD, Hurschler C, Stukenborg-Colsman C, et al. Effect of high tibial flexion osteotomy on cartilage pressure and joint kinematics: A biomechanical study in human cadaveric knees. Winner of the AGA-DonJoy Award 2004. Arch Orthop Trauma Surg 2004;124(9):575–584. DOI: 10.1007/s00402-004-0728-8.

13. Wallace ME, Hoffman EB. Remodelling of angular deformity after femoral shaft fractures in children. J Bone Joint Surg Br 1992;74(5):765–769. DOI: 10.1302/0301-620X.74B5.1527131.

14. McClure PK, Herzenberg JE. The natural history of lower extremity malalignment. J Pediatr Orthop 2019;39(6, Suppl 1):S14–S19. DOI: 10.1097/BPO.0000000000001361.

15. Standard SC. Chapter 1: Normal limb alignment. In: Chase AE, ed. The Art of Limb Alignment. 8th edition. Baltimore, MD: Rubin Institute for Advanced Orthopedics, Sinai Hospital of Baltimore; 2019, pp. 1–16.

16. Paley, D. Chapter 6: Sagittal plane deformities. In: Herzenberg JE, ed. Principles of Deformity Correction. Berlin, Germany: Springer; 2002.

17. Solomin LN, Utekhin AI, Vilenskiy VA. Reference values of the femur and tibia mechanical axes and angles in the sagittal plane, determined on the basis of three-dimensional modeling. J Limb Lengthen Reconstr 2020;6(2):116–120. DOI: 10.4103/2455-3719.305861.

18. McClure PK, Standard SC, Assayag MJ. Chapter 5: Deformity analysis in the sagittal plane. In: Chase AE, ed. The Art of Limb Alignment with Excerpts from the Baltimore Limb Deformity Course Workbook. 9th edition. Baltimore, MD: Rubin Institute for Advanced Orthopedics, Sinai Hospital of Baltimore; 2020, p. 63.

19. Vilenskiy VA., Solomin LN, Utekhin AI. Femur deformity correction planning in sagittal plane using mechanical axis. J Limb Lengthen Reconstr 2021;7(1):13–18. DOI: 10.4103/jllr.jllr_14_21.

20. Jud L, Andronic O, Vlachopoulos L, et al. Mal-angulation of femoral rotational osteotomies causes more postoperative sagittal mechanical leg axis deviation in supracondylar than in subtrochanteric procedures. J Exp Orthop 2020;7(1):46. DOI: 10.1186/s40634-020-00262-6.