Unifying Articular Theory Research Articles

Cubital Tunnel Syndrome Due to Intraneural Ganglion Cysts of the Ulnar Nerve With Joint-Cyst Connection at the Elbow

BACKGROUND AND OBJECTIVES: The unifying articular theory suggests that intraneural ganglion cysts in the cubital tunnel arise from the elbow joint and are connected to the ulnar nerve through an articular branch. This study aimed to report our clinical experience with these cysts and our surgical findings and outcomes. METHODS: We retrospectively analyzed 13 patients who underwent surgery for cubital tunnel syndrome caused by an intraneural ganglion cyst of the ulnar nerve. Clinical symptoms, physical examination findings, nerve conduction testing electromyography, plain radiography, and ultrasonography were evaluated before and after surgery. MRI was performed in 2 patients. Cyst decompression, disconnection of the articular branch, and ulnar nerve transposition were performed in all. RESULTS: The mean follow-up was 29 months. After surgery, local elbow pain and Tinel sign disappeared in all patients, and the average Numerical Rating Scale dropped from 3.7 (2-5) to 0 (0-0). Two-point discrimination testing was normal (<5.0 mm) in 12 patients. Wartenberg sign and “claw hand” deformity became negative, and muscle function and motor nerve conduction velocity were improved in 12 patients. The Medical Research Council muscle strength grade was M4 or M5 in the flexor carpi ulnaris, flexor digitorum profundus of the 4th and 5th digits, and first dorsal interosseous muscle in 12 patients. Postoperative spontaneous potentials including fibrillation and sharp waves, and motor unit recruitment in the first dorsal interosseous muscle were obviously improved. A cystic articular branch was confirmed during surgery in all patients. No cyst recurred clinically or on follow-up ultrasonography. CONCLUSION: The unifying articular theory of intraneural ganglion cysts applies to those located in the ulnar nerve at the elbow. Surgical disconnection of the articular nerve branch from the ulnar nerve eliminated the pathway for cyst recurrence and achieved good long-term outcomes.

Intraneural Mucoid Cysts of Sciatic and Common Fibular Nerve: A Case Report

Intraneural mucoid cyst is uncommonand benign condition, it can result in nerve compressionatlate stages. However, it is a reversible mono-neuropathy disease if diagnosed early and surgically treated [1]. There are numerous hypotheses of pathogenesis such as recurrent trauma, intra-neural hemorrhage, mucoid degeneration and the more recent unified articular theory suggesting that intraneural cysts originates from adjacent joint and dissects along the articular branch into the principal nerve [2].

Acute-Onset Footdrop Caused by Intraneural Ganglion Cyst of the Common Peroneal Nerve: The Effects of Extraneural Pressure Gradients on Cyst Propagation

Ganglion cysts are relatively common entities, but intraneural ganglia within peripheral nerves are rare and poorly understood. We present a case of a 51-year-old man who presented with acute left dropfoot. Initial magnetic resonance imaging (MRI) was misinterpreted as common peroneal neuritis consistent with a traction injury corroborated by the patient's history. However, after surgical decompression and external neurolysis were performed, the patient's symptoms worsened. Repeated MRI revealed an intraneural ganglion cyst of the common peroneal nerve with connection to the superior tibiofibular joint by means of its anterior recurrent branch that was evident retrospectively on preoperative MRI. It is crucial to carefully inspect atypical cases to further recognize and appreciate the dynamic aspect of this disease or "roller-coaster" phenomenon. Intraneural ganglion cysts rely heavily on intraneural and extraneural pressure gradients for propagation, which can be drawn from the expanded work of the unifying articular theory. This report emphasizes the importance of understanding the pathoanatomical and hydraulic factors to appropriately identify and treat intraneural ganglion cysts. Increased recognition of this pathologic entity as a differential diagnosis for acute onset dropfoot is also highlighted.

In Reply: Surgical Treatment of Intraneural Ganglion Cysts of the Ulnar Nerve at the Elbow: Long-Term Follow-up of 9 Cases.

Background Intraneural ganglion (IG) cysts have been considered curiosities and their pathogenesis remains controversial. Objective To clarify ulnar nerve at the elbow (UNE) pathogenesis and long-term surgical outcomes by presenting 9 rare cases of IG of the UNE. Methods Surgical treatment of IG was performed. Clinical symptoms, physical examinations, and electromyogram were evaluated pre- and postoperatively. At least 4 yr of follow-up was performed. Results The Tinel's sign became negative and local elbow pain disappeared in all 9 patients after surgery, and the average visual analog scale/score dropped from 4.9 (3-8) to 0 (0-0) after 6.2 d (2-10) on average. Two patients retained positive Froment test, "claw hand" and paresthesias with the 2-point discrimination much different from the contralateral little finger. Postoperative the UK Medical Research Council muscle strength score (MRC) grades of the flexor carpi ulnaris and the flexor digitorum profundus muscle of the fourth and fifth digits recovered to M4-M5 from M0-M2 in all 9 patients. The postoperative MRC grades of the third to fourth lumbrical muscles, the interossei, and the hypothenar recovered to M3-M5 from M0-M2 in 7 patients. Cystic articular branch (CAB) was found in all 9 patients intraoperatively. No symptomatic recurrence of IG was seen. The mean motor nerve conduction velocity of ulnar nerve across the elbow recovered from 5.3 to 41.2 m/s. Conclusion A unifying articular theory is responsible for the pathogenesis of IG of UNE and disconnection of the CAB would prevent recurrence. The long-term outcome is good after surgical treatment of IG of UNE.

Surgical Treatment of Intraneural Ganglion Cysts of the Ulnar Nerve at the Elbow: Long-Term Follow-up of 9 Cases.

Intraneural ganglion (IG) cysts have been considered curiosities and their pathogenesis remains controversial. To clarify ulnar nerve at the elbow (UNE) pathogenesis and long-term surgical outcomes by presenting 9 rare cases of IG of the UNE. Surgical treatment of IG was performed. Clinical symptoms, physical examinations, and electromyogram were evaluated pre- and postoperatively. At least 4 yr of follow-up was performed. The Tinel's sign became negative and local elbow pain disappeared in all 9 patients after surgery, and the average visual analog scale/score dropped from 4.9 (3-8) to 0 (0-0) after 6.2 d (2-10) on average. Two patients retained positive Froment test, "claw hand" and paresthesias with the 2-point discrimination much different from the contralateral little finger. Postoperative the UK Medical Research Council muscle strength score (MRC) grades of the flexor carpi ulnaris and the flexor digitorum profundus muscle of the fourth and fifth digits recovered to M4-M5 from M0-M2 in all 9 patients. The postoperative MRC grades of the third to fourth lumbrical muscles, the interossei, and the hypothenar recovered to M3-M5 from M0-M2 in 7 patients. Cystic articular branch (CAB) was found in all 9 patients intraoperatively. No symptomatic recurrence of IG was seen. The mean motor nerve conduction velocity of ulnar nerve across the elbow recovered from 5.3 to 41.2 m/s. A unifying articular theory is responsible for the pathogenesis of IG of UNE and disconnection of the CAB would prevent recurrence. The long-term outcome is good after surgical treatment of IG of UNE.

Obturator Intraneural Ganglion Cysts: Joint Connected and Underdiagnosed

Intraneural ganglion cysts of the obturator nerve are rare. Our aim is to review cases of obturator intraneural ganglion cysts at our institution and those reported in the literature. We reviewed all cases evaluated by the senior author. A literature search was performed using the PubMed database and Google Scholar with the following terms: "obturator cyst," "obturator intraneural ganglion cyst," and "obturator intraneural ganglia." All cases underwent a retrospective review. Patient demographic data, including age, sex, and presenting signs and symptoms were recorded. Imaging studies were re-evaluated by 2 musculoskeletal radiologists experienced in the diagnosis of intraneural ganglion cysts. We identified 2 cases of obturator intraneural ganglia at our institution; both were connected to the hip joint. We found 4 cases that were clearly diagnosed as intraneural ganglia in the literature, of which only 1 was recognized by the original authors as being joint connected, but based on our reinterpretation, 3 of 4 were joint connected. An additional 9 cases identified in the literature did not definitely report the nerve-cyst relationship, but based on our reinterpretation, were believed to be intraneural; 8 were joint connected. We believe that obturator intraneural ganglion cysts adhere to the principles of the unifying articular theory. They arise from the anteromedial hip joint and extend into an articular branch and can reach the parent obturator nerve. Surgery should address the hip disease and/or the articular branch connection. Not appreciating the pathoanatomy of these cysts can lead to persistent or recurrent cysts.

Degenerative subtalar joints complicated by medial plantar intraneural cysts : cutting the cystic articular branch prevents recurrence.

The pathogenesis of intraneural ganglion cysts is controversial. Recent reports in the literature described medial plantar intraneural ganglion cysts (mIGC) with articular branches to subtalar joints. The aim of the current study was to provide further support for the principles underlying the articular theory, and to explain the successes and failures of treatment of mICGs. Between 2006 and 2017, five patients with five mICGs were retrospectively reviewed. There were five men with a mean age of 50.2 years (33 to 68) and a mean follow-up of 3.8 years (0.8 to 6). Case history, physical examination, imaging, and intraoperative findings were reviewed. The outcomes of interest were ultrasound and/or MRI features of mICG, as well as the clinical outcomes. The five intraneural cysts followed the principles of the unifying articular theory. Connection to the posterior subtalar joint (pSTJ) was identified or suspected in four patients. Re-evaluation of preoperative MRI demonstrated a degenerative pSTJ and denervation changes in the abductor hallucis in all patients. Cyst excision with resection of the articular branch (four), cyst incision and drainage (one), and percutaneous aspiration/steroid injection (two) were performed. Removing the connection to the pSTJ prevented recurrence of mIGC, whereas medial plantar nerves remained cystic and symptomatic when resection of the communicating articular branch was not performed. Our findings support a standardized treatment algorithm for mIGC in the presence of degenerative disease at the pSTJ. By understanding the pathoanatomic mechanism for every cyst, we can improve treatment that must address the articular branch to avoid the recurrence of intraneural ganglion cysts, as well as the degenerative pSTJ to avoid extraneural cyst formation or recurrence. Cite this article: Bone Joint J 2018;100-B:183-9.

Clinical anatomy leading the way for solutions: An important paradigm for translational research

Clinical anatomy leading the way for solutions: An important paradigm for translational research

Intraneural Ganglions of the Hand and Wrist: Level 4 Evidence

Intraneural ganglions of the upper extremity are rare and they usually originate within the epineurium of the peripheral nerve as nonneoplastic mucinous cysts. Initially, most authors have supported the de novo theory of formation. However, recently, a unifying articular (synovial) theory has been proposed suggesting that the intramural ganglions have connections to the adjacent joints via the articular branches of the involved nerve.

An historical perspective on ulnar intraneural ganglion cysts and their joint origins.

An historical perspective on ulnar intraneural ganglion cysts and their joint origins.

Peripheral neuropathy caused by joint-related cysts: a review of 17 cases

Clinical compression neuropathy caused by para-articular cysts is rare. Only recently, the unifying articular theory was proposed to clarify its true etiologic nature. The authors attribute 17 cases to this theory in order to illustrate the shift in the diagnostic and treatment protocol, and the possible impact on patient outcome. Eight intraneural and nine extraneural cysts were included. The proposed diagnostic protocol includes electromyography and ultrasound, followed by magnetic resonance imaging to characterize the cyst. The proposed treatment protocol consists of (1) ligation of the pedicle connecting the cyst with the afflicted joint, (2) decompression of the nerve and, when needed and (3) disarticulation of the superior tibiofibular joint (in case of peroneal nerve involvement). Outcome was good to excellent in all patients, with recovery of sensory and motor function. Cyst recurrence was observed in three intraneural cases (18 %). Analysis of our own diagnostic protocol showed that atypical compression neuropathies should follow a strict diagnostic protocol to exclude missing the presence of a cyst. Ultrasound needs to play a crucial role, with MRI for cyst characterization and pedicle identification. Retrospective proof in favor of the articular theory was found in all cases. An explanation for the cyst recurrences was formed based on the articular theory. In addition, a diagnostic and therapeutic protocol is proposed for all atypical peripheral compression neuropathies with the ultimate goal to achieve optimal patient outcome.

Evidence to support that adventitial cysts, analogous to intraneural ganglion cysts, are also joint‐connected

Cystic adventitial disease (CAD) is a rare condition in which cyst is found within a vessel, typically producing symptoms of vascular compromise. Most commonly located in the popliteal artery near the knee, it has been reported in arteries and veins throughout the body. Its pathogenesis has been poorly understood and various surgical approaches have been recommended. We extrapolated some recent information about a similar condition, intraneural ganglion cyst affecting the deep fibular (peroneal) nerve, to the prototype, CAD of the popliteal artery. In intraneural ganglion cysts affecting the deep fibular nerve we have shown that an articular (neural) branch is the conduit between the superior tibiofibular joint and the main parent nerve for which epineurial dissection of joint fluid can occur. We hypothesized that the same principles would apply to CAD and that an articular (vascular) branch would be the conduit from the knee joint leading to dissection to the main parent vessel. We reviewed five patients with CAD of the popliteal artery in whom MRIs were available: two treated by the primary author well familiar with the proposed articular theory, and three treated by others at our institution, less familiar with it. We then reviewed the literature critically to assess for additional evidence to support our articular (synovial) theory and an anatomic explanation. In the two cases treated by the primary author a joint connection was identified on high resolution MRI prospectively and intraoperatively through the middle genicular artery (MGA); postoperatively in these cases there was no recurrence. In the other three cases, a joint connection was not identified on imaging or at operation. Reinterpretation of these cases revealed a joint connection through the MGA in the one patient who had preoperative imaging and subclinical persistence/recurrence in the two patients who underwent postoperative MRIs done for other reasons. Our review of the literature and imaging studies revealed unrecognized joint connections in CAD to the knee and other joints as well as evidence that the MGA is the conduit in cases of CAD of the popliteal artery. We believe that adventitial cysts originate in neighboring joints and dissect within articular (vascular) branches. In our opinion, the unifying articular theory and the principles introduced for intraneural ganglion cysts apply equally to common and rare sites of adventitial cysts.

Deep ulnar intraneural ganglia in the palm

While extraneural ganglion cysts are common and well known, intraneural ganglia are rare and misunderstood. We describe a patient with an intraneural ganglion in an unusual location, the deep branch of the ulnar nerve in the palm. We confirmed a connection to the triquetral-hamate joint on preoperative high-resolution MRI and intraoperatively, and observed distal extension of the cyst, a variant pattern of propagation. We wondered if these intraneural cysts followed the principles of the unifying articular (synovial) theory rather than the de novo (degenerative) theory suggested by others. We reviewed patients with ulnar intraneural ganglia at the wrist for joint connections and the pattern of propagation. A total of 35 cases of ulnar intraneural ganglia at the wrist were identified, of which only 10 were joint connected. In 14 cases involving the deep ulnar branch, only 4 had joint connections. We hypothesized and proved that an unrecognized joint connection would be identified in the most recently reported case of a deep ulnar intraneural cyst in which a joint connection had not been identified. Propagation patterns supported descent in all cases involving the deep branch and proximal ascent in those of the main ulnar nerve (n = 18) or the dorsal cutaneous branch (n = 3). We believe that the orientation of the articular branches may play an important role in directionality in these intraneural cysts. Contrary to popular opinion, our analysis of the literature would suggest that intraneural ganglia at this rare site obey the common principles of the articular theory described at more common sites for intraneural ganglia.

Sural Intraneural Ganglion Cysts Are Joint-related

The recent article by Kim and Kim [1] provides a valuable case of a rare sural intraneural ganglion cyst to the literature. We offer yet another example to highlight recent developments in our understanding of this entity. A 42-year-old woman presented with a 6 month history of the spontaneous onset of moderately severe pain developing in the right lateral rearfoot in the retrolateral malleolar region with radiating dysesthesias into the dorsolateral foot. Symptoms were worse with standing or with activity. Exam revealed a 3×2 cm soft tissue tubular mass along the course of the sural nerve which had features consistent with a ganglion cyst. It was tender to palpation. Percussion produced radiating paresthesias in the sural nerve distribution. Sensation was intact on the dorsal foot to light touch. Magnetic reconance imaging (MRI) demonstrated a sural intraneural ganglion cyst with a joint connection to the subtalar joint (Figs. 1, ​,2).2). At operation, this articular branch connection was identified, ligated and resected. The patient had full recovery immediately postoperatively; there was no clinical evidence of persistent pain or cyst at last follow-up examination. Fig. 1 Conventional MRI of a patient with sural intraneural ganglion originating from subtalar joint. Serial coronal T2-weighted fast spin echo (FSE) images with fat suppression from posterior to anterior demonstrates an intraneural cyst arising from the lateral ... Fig. 2 Three-dimensional rendering of conventional magnetic resonance images obtained in this patient with a sural intraneural ganglion cyst. Maximum intensity projection image created from the same series as Fig. 1 shows the entire course of the cyst (asterisk) ... As Kim and Kim [1] point out, the pathogenesis of these cysts has been controversial and poorly understood. Recent reports have clarified this. Layers of robust evidence have substantiated an articular origin for all types of intraneural cysts first at the prototypic site (the common fibular nerve) and then at other joints throughout the appendicular skeleton and axial spine [2,3]. The unifying articular (synovial) theory has been shown to be generalizable and universal. Joint connections must be assumed to be present. We have emphasized the difficulties in identifying the joint connection with imaging and at operation. Experience with both can reliably and reproducibly identify the joint connection. High resolution imaging (MRI or computed tomography imaging with or without arthrography; or ultrasound) with conventional or 3-dimensional renderings may show convincingly a seemingly occult joint connection [3,4], even when reported by others not to be present. The consistent articular connection has fostered an understanding of the principles of these cysts. A capsular defect allows the passage of joint fluid to pass from the joint (often degenerative) along an articular branch and into the parent nerve. Propagation follows the pass of least resistance within the epineurium of nerve. Intra-articular pressure and pressure fluxes influence cyst dimensions. We have identified 9 cases of sural intraneural ganglia (Appendix). Of these, joint connections were only found in 3 instances: twice to the subtalar joint and once to the calcaneocuboid joint. Nevertheless, we are confident that joint connections were present in all of the remaining cases in the literature, including the one by Kim and Kim [1]. These joint connections can be predicted and must be anticipated. The articular branch anatomy of the sural nerve [5] would explain all of the locations of these cysts (Fig. 3). The unifying articular theory could explain all of their patterns of propagation [2,3]. Fig. 3 The anatomy and pathoanatomy of known and potential sites of sural intraneural ganglion cysts. The drawing of the normal anatomy was based on descriptions by anatomists [5]. The confirmed joint-related sites of origin (subtalar and calcaneocuboid joints) ... Understanding the pathogenesis can help target surgery so as to simplify it. Surgery should include ligating or resecting the articular branch near the joint. The cyst itself need not be resected as it will be resorbed as long as the connection is disrupted. This approach minimizes intraneural dissection and eliminates intraneural recurrence, which is known to occur with high rates and was documented in several of these sural examples (Appendix). We believe that such recurrence would likely have been present in other patients (even those who were asymptomatic) if postoperative imaging had been performed. Resecting the nerve, while possible in the case of a distal cutaneous nerve (such as the sural nerve) and has been performed in several cases, could still result in intraneural (distal) recurrence. Furthermore, nerve sacrifice can not be extrapolated to other cases of intraneural ganglia involving major mixed peripheral nerves. Knowledge of current pathogenesis and experience with the imaging and operative findings with these rare cysts will improve outcomes and eliminate intraneural recurrences.

Peroneal and Tibial Intraneural Ganglion Cysts in the Knee Region

Recent research has resulted in an improved understanding of the pathogenesis and treatment of intraneural ganglia, particularly with respect to the most common form, the peroneal nerve at the fibular neck region. To outline the mechanism for the development and propagation of intraneural ganglia located in the knee region, along with their treatment, as well as highlight how shared principles can be exploited for successful treatment of the more commonly occurring peroneal intraneural ganglia. A surgical approach has been developed for peroneal intraneural cysts based on the pathogenesis. The treatment of the less common tibial intraneural cysts is designed along the same principles. A strategy consisting of (1) disarticulation (resection) of the superior tibiofibular joint (ie, the source), (2) disconnection of the articular branch connection (ie, the conduit), and (3) decompression (rather than resection) of the cyst has improved outcomes and eliminated intraneural recurrences in peroneal intraneural cysts. These same principles and techniques can be applied to the rarer tibial intraneural ganglia derived from the same joint. The mechanism of development and propagation for intraneural cysts in the knee region as well as a surgical technique and its rational are described and illustrated. Understanding the joint-related basis of intraneural cysts leads to simple targeted surgery that addresses the joint, its articular branch, and the cyst. The success of the shared surgical strategy for both peroneal and tibial intraneural ganglia confirms the principles of the unifying articular theory.

Extreme intraneural ganglion cysts

The mechanism responsible for exceptional examples of intraneural ganglia with extensive longitudinal involvement has not been understood. Such cases of intraneural cysts, seemingly remote from a joint, have been thought not to have articular connections. Decompression and attempted resection of the cyst has led to intraneural recurrence and poor neurological recovery. The purpose of this report is not only to clarify the pathogenesis of these cysts, but also to discuss their treatment based on modern concepts of intraneural ganglia. Two examples of extreme longitudinal propagation of intraneural ganglia are presented. A patient with a moderate tibial neuropathy was found to have a tibial intraneural ganglion. Prospective interpretation of the MR imaging study demonstrated the cyst's origin from the posterior portion of the superior tibiofibular joint (STFJ), with proximal extension within the sciatic nerve to the lower buttock region. Communication between the STFJ and the cyst was confirmed with direct knee MR arthrography. The tibial intraneural cyst was treated successfully by a relatively limited exposure in the distal popliteal fossa: the cyst was decompressed, the articular branch disconnected, and the STFJ resected. Postoperatively, the patient improved neurologically and there was no evidence of recurrent cyst on postoperative MR imaging. A second patient, previously reported by another group, was reexamined 22 years after surgery. This patient had an extensive peroneal intraneural ganglion that extended into the sciatic nerve from the knee to the buttock; no joint connection or recurrent cyst had initially been described. In this patient, the authors hypothesized and established with MR imaging the presence of both: a joint connection to the anterior portion of the STFJ from the peroneal articular branch as well as recurrent cyst within the peroneal and tibial nerves. This paper demonstrates that extreme intraneural cysts are not clinical outliers but represent extreme examples of other more typical intraneural cysts. They logically obey the same principles, previously described in the unified articular (synovial) theory. The degree of longitudinal extension is probably due to high intraarticular pressures within the degenerative joint of origin. The generalizability of the mechanistic principles is highlighted by the fact that these 2 cases, involving the tibial and the peroneal nerve respectively, both extended well distant (that is, to the buttock) from the STFJ via their respective articular branch of origin. These extensive intraneural cysts can be treated successfully by disconnecting the affected articular branch and by resection of the joint of origin, rather than by a more aggressive operation resecting the cyst and cyst wall.

Evidence that atypical juxtafacet cysts are joint derived

Juxtafacet cysts (JFCs) in usual locations have recently been shown to have joint connections. The pathogenesis of JFCs in unusual locations has remained obscure. The authors hypothesize that all JFCs, including atypical ones, are joint derived. In this study the authors sought to explain the occurrence and formation of clinical outliers of spinal JFCs. In Part I, they performed an extensive literature search to identify case reports of spinal intraneural cysts that have been unappreciated despite the fact that they should occur. In Part II, they studied far-lateral (extraforaminal) cysts treated at their institution and reported in the literature. The presence of a joint connection was specifically looked for since this finding has not been widely appreciated. In Part I, 3 isolated case reports of spinal intraneural JFCs without reported joint connections were identified: 2 involving L-5 and 1, C-8. In Part II, 6 cases involving patients with far-lateral JFCs treated at the authors' institution were reviewed and all 6 had joint connections. Two of these cases had been previously published, although their joint connections were not appreciated. In 2 of the newly reported cases, arthrography confirmed a communication between the facet and the cyst. Only 1 of 5 cases in the literature had a recognized joint connection. The authors believe that all JFCs are joint derived. This explanation for intraneural and extraneural JFCs in typical locations would be consistent with the unified articular (synovial) theory and the pathogenesis for intraneural and extraneural ganglion cyst formation in the limbs. Facet joints appear no different from other synovial joints occurring elsewhere. Understanding the pathogenesis of these cysts will help target treatment to the joint, improve surgical outcomes, and decrease recurrences.

THE UNIFYING ARTICULAR (SYNOVIAL) ORIGIN OF INTRANEURAL GANGLIA

The pathogenesis of intraneural ganglia has been an issue of curiosity, controversy, and contention for 200 years. Three major theories have been proposed to explain their existence, namely, 1) degenerative, 2) synovial (articular), and 3) tumoral theories, each of which only partially explains the observations made by a number of investigators. As a result, differing operative strategies have been described; these generally meet with incomplete neurological recoveries and high rates of recurrence. Recent advances in magnetic resonance imaging and critical analysis of the literature have clarified the mechanisms underlying the formation and propagation of these cysts, thereby confirming the unifying articular (synovial) theory. By identifying the shared features of the typical cases and explaining atypical examples or clinical outliers, several fundamental principles have been described. These include: 1) a joint origin; 2) dissection of fluid from that joint along an articular nerve branch, extension occurring via a path of least resistance; and 3) cyst size, extent, and directionality being influenced by pressures and pressure fluxes. We believe that understanding the pathogenesis of these cysts will be reflected in optimal surgical approaches, improved outcomes, and decreased frequency, if not elimination, of recurrences. This article describes the ongoing process of critically analyzing and challenging previous observations and evidence in an effort to prove a concept and a theory.

Intraneural ganglia in the hip and pelvic region

The authors describe their experience in a series of cases of intraneural ganglia within the hip and pelvic regions, and explain the mechanism of formation and propagation of this pathological entity. Five patients with 6 intraneural ganglia are presented. Four patients presented with symptomatic intraneural ganglia in the buttock and pelvis affecting the sciatic and lumbosacral plexus elements. An asymptomatic cyst affecting the opposite sciatic nerve was found on MR imaging in 1 patient. The fifth patient, previously reported on by another group, had an obturator intraneural ganglion that the authors reinterpreted. All 5 intraneural ganglia affecting the sciatic and lumbosacral plexus elements were found to have a joint connection to the posteromedial aspect of the hip joint; the obturator intraneural cyst had a joint connection to the anteromedial aspect of the hip joint. In all cases, initial review of the MR images led to their misinterpretation. To the authors' knowledge, these are the first cases of intraneural ganglia demonstrated to have a connection to the hip joint. This finding at a rare site provides further evidence for the unifying articular (synovial) theory for the formation of intraneural ganglia and reveals a shared mechanism for their propagation. Furthermore, understanding the pathogenesis of these lesions provides insight into their successful treatment and their recurrence.

Adherence of intraneural ganglia of the upper extremity to the principles of the unifying articular (synovial) theory

Intraneural ganglia are nonneoplastic mucinous cysts contained within the epineurium of peripheral nerves. Their pathogenesis has been controversial. Historically, the majority of authors have favored de novo formation (degenerative theory). Because of their rarity, intraneural ganglia affecting the upper limb have been misunderstood. This study was designed to critically analyze the literature and to test the hypothesis that intraneural ganglia of the upper limb act analogously to those in the lower limb, being derived from an articular source (synovial theory). Two patients with digital intraneural cysts were included in the study. An extensive literature review of intraneural ganglia of the upper limb was undertaken to provide the historical basis for the study. In both cases, the digital intraneural ganglia were demonstrated to have joint connections; the one patient in whom an articular branch was not appreciated initially had evidence on postoperative MR images of persistence of intraneural cyst after simple decompression was performed. Eighty-six cases of intraneural lesions were identified in varied locations of the upper limb: the most common sites were the ulnar nerve at the elbow and wrist, occurring 38 and 22 times, respectively. Joint connections were present in only 20% of the cases published by other groups. The authors believe that the fundamental principles of the unifying articular (synovial) theory (that is, articular branch connections, cyst fluid following a path of least resistance, and the role of pressure fluxes) previously described to explain intraneural ganglia in the lower limb apply to those cases in the upper limb. In their opinion, the joint connection is often not identified because of the cysts' rarity, radiologists' and surgeons' inexperience, and the difficulty visualizing and demonstrating it because of the small size of the cysts. Furthermore, they believe that recurrence (subclinical or clinical) is not only underreported but also predictable after simple decompression that fails to address the articular branch. In contrast, intraneural recurrence can be eliminated with disconnection of the articular branch.