PTTD And Spring Ligament Failure: The Chicken And The Egg?

A recent study of tendon and ligament loads in the foot adds to the debate about the sequence of events leading to adult-acquired flatfoot deformity (AAF).¹ This new study, conducted by De la Portilla and coworkers, used finite element computer modeling to study the relationship of load sharing among the key soft tissue supports of the medial arch and hindfoot.

The study revealed that when the spring ligament is deficient, there is significant stress on the posterior tibial tendon (PTT).1 When evaluating the reverse scenario, less significant load transfer to the spring ligament occurs when the posterior tibial tendon is deficient. The authors suggest that spring ligament failure may be the primary anatomic failure in adult-acquired flatfoot. This study provides caution to practitioners who may focus solely on treating the posterior tibial tendon while overlooking the underlying causative role of adult-acquired flatfoot, which is spring ligament failure.

This new study reinforces the insight gained from a previous study conducted by Jennings and Christensen.² The authors sectioned the spring ligament in five cadaveric specimens subjected to axial load while also loading the posterior tibial tendon. When the spring ligament was sectioned, a significant change of alignment associated with flatfoot deformity occurred which the posterior tibial tendon was unable to compensate.

The longstanding accepted mechanical model of adult-acquired flatfoot is that attenuation and rupture of the posterior tibial tendon are the primary cause of the deformity.^3,4 In fact, the Johnson and Strom classification, which is still the most common staging for adult-acquired flatfoot, is based solely upon the integrity of the posterior tibial tendon with no mention of ligament injury.⁵

Yet there is increasing evidence that the spring ligament is the primary structure that fails in adult-acquired flatfoot. Magnetic resonance imaging (MRI) studies show that spring ligament attenuation and rupture is as common as rupture in stage 2 adult-acquired flatfoot deformity.^6,7 Furthermore, cadaveric studies have shown the intact foot will not collapse and develop the adult-acquired flatfoot deformity unless multiple ligaments rupture, not just the posterior tibial tendon.^8,9

Not only have there been challenges to the role of the posterior tibial tendon in adult-acquired flatfoot deformity but the sequence of events in the pathomechanics is now up for debate. Some authorities have proposed that attenuation of the spring ligament is the primary event that actually precedes posterior tibial tendon rupture.

Pasapula, an orthopedic surgeon based in the United Kingdom, has challenged the notion of posterior tibial tendon dysfunction causing the adult acquired flatfoot.¹⁰ He has appropriately criticized the oft-quoted classification system proposed by Johnson and Strom, and has developed a more appropriate classification based upon spring ligament failure.¹¹ Pasapula and Cutts propose that the synovitis seen with Stage 1 adult-acquired flatfoot is the result of spring ligament failure.¹¹ The authors modified the original Johnson and Strom classification of adult-acquired flatfoot to include a stage 0, which is a condition of laxity in the spring ligament without synovitis of the posterior tibial tendon. One can detect this laxity by a simple clinical test, which they describe as the “neutral heel lateral push test.” A cadaveric study validated this test, demonstrating its ability to detect spring ligament rupture in isolation of posterior tibial tendon rupture.¹²

I have performed the neutral heel lateral push test on many patients with adult-acquired flatfoot and I am convinced it is a valuable tool to detect laxity at the talonavicular joint caused by spring ligament rupture. The challenge is determining what treatment would be necessary in a Stage 0 patient who has laxity of the spring ligament but no symptoms in the posterior tibial tendon? What percentage of Stage 0 patients will progress to synovitis (Stage 1) and eventual rupture of the posterior tibial tendon (Stage 2) in the new Pasapula and Cutts classification system? Finally, is operative intervention to repair the spring ligament indicated earlier than tradition has previously dictated before catastrophic collapse of the foot can occur? This option has become more attractive with the availability of synthetic ligament augmentation repair technology.^10,13,14

Alternatively, it may be important to consider early intervention with functional ankle-foot orthotic (AFO) bracing when one has identified patients with attenuation of the spring ligament. I have written previously about the impressive outcomes with custom AFO bracing in treating patients with Stage 2 adult-acquired flatfoot deformity.^15,16 A significant proportion of patients with adult-acquired flatfoot treated with AFO bracing remain symptom-free for many years. With this outcome, it is assumed that ruptured ligaments have healed just as they do after the treatment of a lateral ankle sprain with functional bracing.^17,18

Whether the posterior tibial tendon and the spring ligament are the chicken or the egg, one should focus attention on spring ligament failure as a critical step in the pathomechanics of adult-acquired flatfoot deformity. Further studies are needed to determine if early intervention in treating spring ligament insufficiency will prevent the progression of adult-acquired flatfoot.

References

1. De la Portilla CC, Larrainzar-Garijo R, Bayod J. Biomechanical stress analysis of the main soft tissues associated with the development of adult acquired flatfoot deformity. Clin Biomech. 2019; 61(1):163–171.
2. Jennings MM, Christensen JC. The effects of sectioning the spring ligament on rear foot stability and posterior tibial tendon efficiency. Foot Ankle Surg. 2008; 47(3):219-224.
3. Myerson MS. Adult acquired flatfoot deformity: Treatment of dysfunction of the posterior tibial tendon. Instr Course Lect. 1997; 46:393-405.
4. Kohls-Gatzoulis J, Angel JC, Singh D, et al. Tibialis posterior dysfunction: a common and treatable cause of adult acquired flatfoot. BMJ. 2004; 329(7478):1328–1333.
5. Johnson KA, Strom DE. Tibialis posterior tendon dysfunction. Clin Orthop Relat Res. 1989; 239:196-206.
6. Deland JT, de Asla RJ, Sung IH, et al. Posterior tibial tendon insufficiency: which ligaments are involved? Foot Ankle Int. 2005;26(6):427-35.
7. Williams G, Widnall J, Evans P, Platt S. Could failure of the spring ligament complex be the driving force behind the development of the adultflatfoot deformity? J Foot Ankle Surg. 2014;53(2):152-5. 
8. Niki HN, Ching RP, Kiser P, Sangeorzan BJ. The effect of posterior tibial tendon dysfunction on hindfoot kinematics. Foot Ankle. 2001; 22(4):292-300.
9. Deland JT, Arnoczky SP, Thompson FM. Adult acquired flatfoot deformity at the talonavicular joint: Reconstruction of the spring ligament in an in vitro model. Foot Ankle. 1992; 13(6):327-332.
10. Pasapula C, Devany A, Hübner T, et al. The resistance to failure of spring ligament reconstruction. Foot. 2017; 27: 81-89.
11. Pasapula C, Cutts S. Modern theory of the development of adult acquired flat foot and an updated spring ligament classification system. Clin Res Foot Ankle. 2017; 5(3):247.
12. Pasapula C, Devany A, Magan A, et al. Neutral heel lateral push test: The first clinical examination of spring ligament integrity. Foot. 2015; 25(2):69-74.
13. Acevedo J, Vora A. Anatomical reconstruction of the spring ligament complex: internal brace augmentation. Foot Ankle Spec. 2013;6(6):441-5.
14.  Palmanovich E, Shabat S, Brin YS et al. Novel reconstruction technique for an isolated plantar calcaneonavicular (SPRING) ligament tear. A 5 case series report. Foot. 2017; 30:1-4.
15. Richie D. Why conservative treatment is the standard of care for adult-acquired flatfoot. Podiatry Today DPM Blog. Available at https://www.podiatrytoday.com/blogged/why-conservative-treatment-standard-care-adult-acquired-flatfoot . Published June 2, 2011.
16. Richie D. Eight tips for conservative treatment of adult-acquired flatfoot. Podiatry Today DPM Blog. Available at https://www.podiatrytoday.com/blogged/eight-tips-conservative-treatment-adult-acquired-flatfoot . Published Nov. 2, 2017.
17. Kannus P, Renstrom P. Current concepts review: treatment for acute tears of the lateral ligaments of the ankle. J Bone Joint Surg Am. 1991;73(2):305–312.
18. Bleakley CM, McDonough SM, MacAuley DC. Some conservative strategies are effective when added to controlled mobilisation with external support after acute ankle sprain: a systematic review. Aust J Physiother. 2008;54(1):7–20.