Myositis of the inferior rectus muscle on Medpor® implant: a diagnostic and surgical challenge

The use of high-density porous polyethylene implants (Medpor®) has become the standard in orbital wall reconstruction, particularly for its ability to promote fibrovascular integration. However, this architecture can, in rare cases, trigger a "foreign body" type immune response or pathological tissue adhesion. This study reports the rare clinical case of a 38-year-old patient who developed isolated myositis of the inferior rectus muscle only one week after orbital floor fracture repair using a Medpor® implant.

The objective of this report is to document the management of diplopia and dull pain refractory to conventional medical treatments (corticosteroids at 1 mg/kg/day and antibiotic therapy). The authors present here a complex surgical revision strategy combining a biplanar transorbital and transantral approach for implant excision and tissue release.

The central hypothesis is based on a mixed etiology: a localized inflammatory reaction induced by the implant associated with mechanical entrapment of the muscle fibers due to incomplete coverage of the bone defect. This case demonstrates that, despite the theoretical biocompatibility of Medpor®, tissue integration can sometimes lead to severe motor restriction requiring the replacement of the synthetic implant with an autologous dermo-fat graft.

Management protocol and study design

This study reports the clinical case of a 38-year-old patient, non-smoker and without systemic history, presenting with isolated myositis of the inferior rectus muscle following the placement of a Medpor® implant for an orbital floor fracture. Diagnostic analysis was based on clinical measurements of visual acuity (logMAR 0.18 right, 0.00 left), Hertel exophthalmometry (18 mm OD, 20 mm OS) and extraocular motility assessment (restriction of -3 in upgaze and -2 in downgaze). Imaging included a CT scan and dynamic multiparametric MRI (cine-T2 sequences) to confirm nodular muscle enlargement.

The therapeutic protocol was carried out in two phases:

• Initial medical treatment: Prescription of oral prednisolone (1 mg/kg/day) combined with antibiotic prophylaxis, which had no effect on the diplopia.
• Surgical revision: Transorbital and transantral biplanar approach under general anesthesia. A 1.5 cm vestibular incision and a maxillary window created with a 6 mm bur allowed transantral access.
• Specific technique: Use of the "Kangaroo Pouch" method, consisting of a submucosal dissection via gentle pressure exerted by a periosteal dissector on neurosurgical compresses soaked in tranexamic acid.
• Reconstruction: Removal of the implant via a transconjunctival incision of the inferior fornix, followed by an autologous dermofat graft harvested from the anterosuperior iliac crest and impregnated with 5-fluorouracil.

The postoperative follow-up extended over 6 months to evaluate the resolution of the vertical diplopia.

Clinical results and intraoperative observations

The initial evaluation of this 38-year-old patient revealed marked functional and structural deficits following the initial placement of the Medpor® implant. Objective clinical measurements and imaging observations allowed for the characterization of the inflammatory and mechanical involvement.

Preoperative clinical and functional data

The following table summarizes the clinical parameters recorded when the patient presented to the institution (3 weeks after the initial surgery):

Serological tests showed a thyroid-stimulating immunoglobulin (TSI) level < 89 and a positive antinuclear antibody (ANA) test. The forced duction test was positive in upward traction for the left eye, confirming a mechanical restriction.

Observations by multiparametric imaging

• CT Scan (CT): Evidence of an isolated nodular enlargement of the left inferior rectus muscle.
• Dynamic MRI (T2 cine sequences): Confirmation of T2 hyperintensity and focal enlargement of the inferior rectus muscle body. Dynamic sequences revealed entrapment of adjacent soft tissues due to incomplete coverage of the bone defect by the implant, physically limiting globe elevation.

Surgical findings and postoperative follow-up

L'approche transantrale a permis de visualiser une muqueuse sinusale cicatrisée mais a révélé, après dissection, une zone de non-couverture médiale de l'implant Medpor® laissant les fibres du muscle droit inférieur exposées et adhérentes.

After the removal of the implant and the placement of an autologous dermo-fat graft (impregnated with 5-fluorouracil), the clinical evolution showed a progressive improvement:

• At 1 week: Ocular alignment was straight in primary position. Diplopia persisted in upward gaze, with an improvement in motility (excursions at -1 upward and -2 downward).
• At 6 months: The authors report a complete resolution of the vertical diplopia, with only a minor residual limitation of -1 remaining in upward gaze.

Clinical analysis: When the implant becomes the aggressor

This case highlights a rare but severe complication of porous polyethylene implants (Medpor®): isolated myositis of the inferior rectus muscle. Although this material has been favored for its stability and fibro-integration for three decades, it can induce a foreign body immune response. Here, the symptoms (dull pain, diplopia) that appeared early revealed focal inflammation refractory to corticosteroids and antibiotics.

Differential diagnosis is crucial. Although thyroid disease or idiopathic myositis were considered, the temporal correlation with implant placement and imaging pointed towards an iatrogenic cause. The contribution of multiparametric MRI with cine-T2 sequences was decisive in identifying muscle hyperintensity and confirming its mechanical incarceration related to incomplete coverage of the fracture by the implant.

The major limitation of this report lies in its isolated nature (n=1). However, it validates the relevance of the biplanar surgical approach (transorbital and transantral). This technique offers superior visualization to release soft tissues and remove the implant, where a single approach might be insufficient. The use of an autologous dermofat graft to buffer the floor defect helps to mitigate the risks of post-removal re-adhesion.

Summary of results

This double-approach procedure (transorbital and transantral) allowed for the excision of the Medpor® implant and the release of the inferior rectus muscle incarceration. The use of an autologous dermis-fat graft, replacing the synthetic material, led to a progressive resolution of diplopia and orbital pain in this 38-year-old patient.

In practical terms, for the practitioner:

• Diagnostic vigilance: Persistent diplopia associated with focal myositis on imaging suggests a possible immune reaction to the implant, even in the absence of systemic autoimmunity signs.
• Surgical precision: The biplanar approach combined with the "Kangaroo Pouch" technique (sinus submucosal dissection) ensures direct and complete visualization of the bone margins, guaranteeing soft tissue release.
• Biological alternative: The autologous dermo-fat graft constitutes a reliable reconstruction solution in cases of revision for intolerance or inflammatory reaction to conventional porous biomaterials.

Technical lexicon of the study

Medpor®: High-density porous polyethylene implant used for the reconstruction of orbital walls and rims, promoting fibrovascular integration thanks to its interconnected architecture.

Inferior rectus muscle myositis: Inflammatory condition specifically affecting the inferior rectus muscle, manifesting as diplopia, motility restriction, and nodular enlargement of the muscle on imaging.

Kangaroo Pouch technique: Method of maxillary mucosa dissection via transantral access allowing exposure of the fracture extent and the orbital floor.

Dermal fat graft: Autologous substitute used as a replacement for the removed Medpor® implant, serving to fill the bone defect and reconstruct the orbital floor.

Transorbital-transantral biplanar approach: Surgical strategy combining transconjunctival access and maxillary sinus access to allow the removal of an implant and the release of incarcerated soft tissues.

T2 Cine-MRI: Multiparametric dynamic imaging sequence used to confirm the restriction of ocular elevation and focal enlargement of the inferior rectus muscle.

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