Phallic Urethra: Tube-in-Tube and Pars Pendulans Construction
The phallic urethra, or pars pendulans, is the distal neourethral segment that runs through the neophallus from its base to the neomeatus. Unlike the pars fixa, which is built from local perineal tissue, the pars pendulans is determined primarily by flap choice and phalloplasty design.[1][2][3]
For full donor-flap anatomy and non-GAS applications, see Radial Forearm Free Flap. For the phalloplasty-specific donor-site page, see RFFF Phalloplasty.
Concept
The neourethra can be divided into two operative segments.[1][4]
| Segment | Location | Main Technical Driver |
|---|---|---|
| Pars fixa | Native urethral meatus to phallus base | Anterior vaginal-wall flap, labia minora / vestibular tissue, buccal mucosa graft, vaginectomy, buttress coverage |
| Pars pendulans | Phallus base to neomeatus at phallus tip | Tube-in-tube RFFF, separate urethral flap, prelaminated flap, or staged graft urethroplasty |
The pars pendulans must be long, hair-free, supple, well vascularized, and positioned so its suture line does not sit directly under the outer shaft closure. It also has to meet the pars fixa without tension; the pars fixa-to-pars pendulans junction remains a major stricture site.[1][3][5] Among 118 urethroplasties in 79 patients already selected for post-phalloplasty stricture repair, 48 (41%) treated the pars fixa–pars pendulans junction, 28 (24%) the phallic urethra, 18 (15%) the meatus, 15 (13%) the fixed urethra, and 9 (8%) multiple sites.[19]
Tube-in-Tube RFFF
The classic tube-in-tube radial forearm free flap (RFFF) creates the phallic urethra and shaft from one fasciocutaneous flap. It is the most commonly reported option for simultaneous shaft, urethral, and sensory reconstruction; forearm skin is thin, pliable, and reliably perfused by the radial artery system. A 2026 systematic review found no gold-standard phalloplasty technique and insufficiently consistent outcomes for meaningful comparison.[2][6][7][24]
| Step | Technical Point |
|---|---|
| Design | A broad volar forearm flap is divided into a narrow urethral strip and a wider shaft segment |
| Inner tube | The urethral strip is rolled skin-side-in around a catheter to create the pars pendulans |
| Outer tube | The remaining flap is rolled around the inner tube, skin-side-out, to create the phallic shaft |
| Vascularity | Inner and outer tubes share the radial artery / venae comitantes and superficial venous drainage |
| Nerve coaptation | Forearm sensory nerves are coapted to dorsal clitoral nerve, ilioinguinal nerve, or other local targets according to center protocol |
| Suture-line planning | Inner and outer suture lines should be offset to reduce overlapping ischemia and fistula risk |
The main strength is that the urethra and shaft travel as one vascularized unit. The phallic urethra can be joined to the pars fixa at the index operation or at a second stage after phallus creation, as in the Big Ben method.[1] The main vulnerability is that the inner skin tube has a long ventral suture line, and any hair-bearing skin left in the urethral strip can become a lifelong obstructive and infectious problem.
Advantages and Tradeoffs
| Advantage | Why It Matters |
|---|---|
| Single-flap urethra and shaft | Avoids a second free flap or staged phallic urethral graft bed |
| Thin pliable tissue | Forearm skin tubularizes more predictably than thick thigh tissue |
| Reliable vascular anatomy | Radial artery anatomy is usually consistent and familiar to microsurgical teams |
| Sensory potential | Forearm sensory nerves support tactile and erogenous reinnervation planning |
| Established outcomes literature | RFFF has the largest phalloplasty evidence base |
| Tradeoff | Counseling Point |
|---|---|
| Visible forearm donor site | The donor scar is often the decisive disadvantage |
| Microsurgery required | Requires reliable microsurgical team, recipient vessels, and postoperative monitoring |
| Urethral suture-line morbidity | Fistula and stricture remain common even when flap survival is good |
| Forearm size and hair pattern matter | Small forearms, thick adiposity, or dense hair can compromise design |
| Radial artery sacrifice | Requires vascular assessment and hand-perfusion safety checks |
Alternatives to Tube-in-Tube RFFF
When RFFF tube-in-tube is not used, the surgeon must choose how the phallic urethra will be supplied. This choice is separate from the shaft flap choice and should be made before committing to donor-site scars or staging.
| Technique | Best Use | Main Limitation |
|---|---|---|
| Second free flap for urethra | ALT shaft with a dedicated RFFF or other thin urethral flap | Adds a second donor site and additional microsurgery |
| SCIP urethral flap | ALT or revision phalloplasty when a thin concealed-donor urethral flap is desired | Requires favorable groin tissue and flap-specific experience |
| Prelaminated ALT | Patient wants ALT shaft and a planned graft-lined urethra | Historically high complication rates from graft take, contracture, and vascular-interface problems |
| Staged split-thickness skin-graft urethroplasty | ALT shaft without reliable tube-in-tube urethra | Requires three-stage pathway and delayed standing voiding |
| Pedicled labia minora flap | Selected ALT / SCIP phalloplasty with usable local genital tissue | Limited length and substantial stricture / fistula rates in early series |
| Tube-in-tube ALT | Rare thin-thigh candidate with favorable hair pattern | Usually not feasible; ALT is often too thick and hair-bearing |
Second Free Flap
Double-flap phalloplasty pairs a pedicled ALT shaft with a separate RFFF used only for urethral reconstruction. Van der Sluis and colleagues reported 19 transgender men undergoing pedicled ALT phalloplasty with RFFF urethral reconstruction; long-term urinary complications occurred in 53%, including strictures in 47%, and total RFFF failure occurred in two patients.[8]
D'Arpa's retrospective mixed-population series included 93 ALT phalloplasties (79 for gender affirmation; seven without urethral reconstruction) and reported fistula or stricture in 11/29 RFFF-urethra and 10/38 SCIP-urethra cases.[9] A second RFFF can also salvage loss of a tube-in-tube urethra after partial flap necrosis, but it spends the contralateral forearm and should be reserved for carefully selected salvage situations.[10]
Prelaminated Flap
Prelamination places a graft onto the planned flap before phalloplasty, allows it to vascularize, and later tubularizes it as the phallic urethra. It is appealing in theory, but the outcome signal has been poor in ALT urethral reconstruction. D'Arpa reported an 87.5% urethral complication rate for prelaminated ALT urethra, and the Hu meta-analysis found prelaminated flaps associated with a significantly higher stricture rate than tube-in-tube approaches and no difference from second-flap techniques.[9][11]
Staged Skin-Graft Urethroplasty
Bluebond-Langner's staged ALT pathway avoids forcing a thick thigh flap into a tube-in-tube urethra. Stage I creates a single-tube ALT phallus without urethra. Stage II opens the ventral phallus, grafts a urethral plate, and completes vaginectomy / pars fixa / scrotoplasty. Stage III tubularizes the grafted plate to complete the pars pendulans.[12]
Robinson and colleagues reported 24 patients treated with staged split-thickness skin-graft ALT urethroplasty: standing micturition was achieved in 87.5% at the time of data collection, with fistulae requiring additional operative intervention in 33.3% and strictures requiring additional operative intervention in 20.8%.[12]
Labia Minora and SCIP Urethral Flaps
Pedicled labia minora flaps can form both fixed and pendulous urethral components when local tissue length is adequate. In the Amsterdam / Belgrade first-experience series of 16 patients undergoing ALT or SCIP phalloplasty with pedicled labia minora flap urethral lengthening, fistula occurred in 25%, stricture in 37.5%, and standing micturition in 56.3%.[13]
SCIP can be used as a dedicated thin urethral flap, especially with ALT shaft reconstruction. In D'Arpa's mixed-population ALT series, fistula or stricture occurred in 10/38 SCIP urethras; a later retrospective gender-affirming series reported complete loss of 8/45 SCIP urethral reconstructions, including one removed at patient preference.[9][26] For flap-specific selection, see SCIP Phalloplasty.
Tube-in-Tube ALT
Tube-in-tube ALT is uncommon because thigh tissue is often thick and hair-bearing. In D'Arpa's 93-case ALT series, tube-in-tube ALT was feasible in only 5 patients, with a 20% urethral complication rate in that highly selected subgroup.[9]
Comparative Outcomes
| Pars Pendulans Strategy | Largest Cited Series / Context | Urethral Signal |
|---|---|---|
| Tube-in-tube RFFF | Largest evidence base; RFFF remains reference standard | Lower stricture risk than prelaminated approaches in meta-analysis, but combined urethral complications remain common[2][7][11] |
| Second RFFF urethral flap with ALT shaft | van der Sluis 2017, 19 patients; D'Arpa 2019, 29 patients | 53% urinary complications / 47% strictures in van der Sluis; 37.9% urethral complications in D'Arpa[8][9] |
| SCIP urethral flap | D'Arpa 2019, 38 patients | 26.3% urethral complication rate in ALT urethral-reconstruction series[9] |
| Staged skin-graft ALT urethroplasty | Robinson 2023, 24 patients | 87.5% standing micturition at data collection; 33.3% fistulae requiring additional operative intervention; 20.8% strictures requiring additional operative intervention[12] |
| Prelaminated ALT urethra | D'Arpa 2019, 8 patients; Hu 2022 meta-analysis | 87.5% urethral complications in D'Arpa; higher stricture rate than tube-in-tube in meta-analysis[9][11] |
| Pedicled labia minora flap | Al-Tamimi 2020, 16 patients | 25% fistula; 37.5% stricture; 56.3% standing micturition[13] |
Complications
| Complication | Mechanism / Pattern | Prevention or Repair Direction |
|---|---|---|
| Urethrocutaneous fistula | Long inner-tube suture line, ischemia, distal obstruction, or overlapping closures | Offset suture lines, ensure vascularized coverage, treat distal obstruction before fistula closure |
| Stricture | Pars fixa-to-pars pendulans anastomosis, neomeatus, graft contracture, or ischemic tube | Define with cystoscopy / contrast imaging; avoid repeated low-yield dilation |
| Intraurethral hair | Hair-bearing donor skin in urethral strip | Preoperative depilation; endoscopic laser epilation for postoperative hair-bearing grafts |
| Neomeatal stenosis | Tip ischemia, scar, glansplasty / meatal configuration | Meatoplasty or staged revision when persistent |
| Urethral loss after partial flap necrosis | Inner tube ischemia or partial flap loss | Salvage with staged graft urethroplasty or second flap in selected cases |
In Veerman and colleagues' retrospective cohort of 63 patients, stricture formed in 63% of phalloplasty patients and fistula in 27%, and 73% overall needed revision.[14] Across the 63-patient cohort (55 phalloplasty and 8 metoidioplasty cases), the mean time to stricture or fistula was about 3 months.[14] Revision algorithms keep the urethra decompressed, identify distal obstruction, and avoid definitive repair through inflamed tissue; advanced revision may require staged grafting, local flaps, or second-flap salvage depending on tissue loss and patient goals.[18] The 2026 EAU guideline strongly recommends delaying endoscopic incision or urethroplasty until at least six months after neophalloplasty and limiting endoscopic incisions to two unless palliation is intended.[27] It weakly recommends staged urethroplasty for a neophallic urethral stricture when open repair is indicated.[27]
Catheterization After Phalloplasty
After phalloplasty, urethral tortuosity can make blind Foley passage difficult; one reconstructive-urology center describes a single 14-Fr coude attempt, with cystoscopic guidewire placement if it fails.[20][28] If catheter passage is difficult, avoid repeated forceful attempts and escalate to flexible cystoscopy with guidewire-assisted Foley placement or suprapubic access when appropriate. In a 20-patient randomized implant-surgery trial, initial placement succeeded in 3/10 patients assigned a standard Foley and 6/10 assigned a dedicated catheter; failed attempts were managed with cystoscopic guidewire placement.[20][25] Patients can carry their reconstructive surgeon's contact information so other treating teams can confirm the neourethral anatomy before instrumentation. This is a practical step when access to the original surgeon may be limited during an emergency.[20]
Hair Removal
Hair-bearing urethral skin can cause recurrent urinary tract infection, obstructive symptoms, encrustation, stone formation, and difficult cystoscopy. Hair clearance must be planned before flap elevation, especially for RFFF and ALT urethral strips.[15][16]
| Modality | Practical Point |
|---|---|
| Electrolysis | Most definitive method for permanent follicle destruction; slow and operator-dependent; treats all hair colors and skin types[30] |
| Laser hair removal | Faster and less painful for dark terminal hair, but produces permanent reduction rather than guaranteed removal; depends on melanin and is not suitable for light hair. Long-pulsed Nd:YAG is an option for Fitzpatrick III–VI skin: in a 12-trial meta-analysis of laser and IPL hair reduction, it had similar hair-count reduction to IPL and less postinflammatory hyperpigmentation.[21][29] |
| Timing | Start early; multiple sessions and a regrowth observation interval are usually needed before surgery |
| Tattooed donor skin | Avoid laser or intense-pulsed-light treatment directly over tattooed skin intended as a graft or urethral donor site: case reports describe ulceration, keloid, or second-degree burn.[22][23] If no non-tattooed donor area is available, electrolysis has been suggested instead.[21] |
| Postoperative hair | Nd:YAG laser epilation via cystoscopy has been described for hair-bearing urethral grafts |
Single-Tube Phalloplasty Without Pars Pendulans
Some patients choose phalloplasty without urethral lengthening and continue voiding from the native perineal meatus. This avoids pars pendulans and pars fixa morbidity but sacrifices standing micturition. Miller and colleagues described a single-tube RFFF modification that creates an aesthetic distal neomeatal pouch without a functional urethral connection, preserving the appearance of a meatus while avoiding urethral lengthening.[17]
Operative Pearls
- Decide whether standing micturition is worth the urethral risk before choosing the shaft flap.
- For tube-in-tube RFFF, plan the urethral strip first.
- Offset inner and outer suture lines so fistula-prone closures do not stack directly over each other.
- Do not force tube-in-tube construction into a thick ALT flap; choose a staged graft or separate flap pathway when tissue is unfavorable.
- Treat prelaminated ALT urethra cautiously; the reported urethral complication rate was 87.5% in D'Arpa's series.
- Always evaluate the pars fixa-to-pars pendulans junction when a patient presents with pendulous urethral symptoms.
- For patients who do not prioritize standing voiding, no-UL phalloplasty is a legitimate morbidity-sparing endpoint.
References
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