BL-33 (Zhong Liao) Central Bone Hole

What does BL-33 Zhong Liao do?

Zhong Liao BL-33 is the third of the four sacral foramen points on each side — the point that lies directly over the third posterior sacral foramen — and its recorded work is to regulate the lower burner and free urination and defecation, to regulate menstruation and stop discharge, and to benefit the lumbar region and legs. Sacred Lotus sources & references record those three actions and also classify it, like its three companions, as a Meeting Point of the Bladder and Gall Bladder channels. Its defining feature is anatomical rather than theoretical: the point sits over an actual opening in the sacrum through which the posterior branch of the third sacral nerve emerges. That is why the sacral points behave less like ordinary channel points and more like access routes.

• Regulates the lower burner and frees urination and defecation — the headline recorded action. The lower burner (xia jiao) is the tradition's name for the lowest division of the trunk, holding the bladder, the intestines below the navel and the reproductive organs. This action is the reason BL-33 dominates the modern acupuncture literature on bladder and bowel dysfunction, and it is the one action of this point with substantial human trial evidence behind it.
• Regulates menstruation and stops leucorrhoea — the gynaecological side: irregular and painful periods, and abnormal vaginal discharge.
• Benefits the lumbar region and legs — for low back and sacral pain, and for pain, numbness or weakness running down the lower limb.
• Reaches the pelvic floor and the pelvic organs from behind — the practical consequence rather than a separately listed action, but the one that explains the point's modern career. The third sacral nerve is a principal supply to the pelvic floor and the bladder, and BL-33 is the point that sits over it.

One point about how these actions should be framed. BL-33 is not a Back-Shu point and should never be described as one — it is not among the twelve points beside the spine that carry an organ's name. Queried directly, our own data record no Back-Shu classification for it at all. Its influence over the pelvic organs comes from its position over a sacral foramen and from the record of its clinical use, which is a different and more literal kind of claim than the Back-Shu doctrine makes. The Back-Shu point of the Bladder is Pang Guang Shu BL-28, held here, which lies further in toward the midline at the second foramen level.

Source: Sacred Lotus sources & references (channel, the Meeting Point classification, and the existing action record — reformatted here from a broken bare list run; the BL-28, BL-31, BL-32 and BL-34 records queried directly); Deadman & Al-Khafaji, A Manual of Acupuncture, p. 294; Chinese Acupuncture & Moxibustion, p. 182; cross-referenced against multiple online sources.

What is BL-33 Zhong Liao used for?

Zhong Liao BL-33 is used for the urinary, bowel and gynaecological complaints of the pelvis, and for low back and sacral pain — and in modern practice above all for urinary incontinence and retention, where it is the sacral point with the best trial evidence of the four. The indications carried in Sacred Lotus sources & references are low back pain, constipation, diarrhoea, difficult urination, irregular menstruation and abnormal vaginal discharge. The standard reference literature sets them out more fully below. These describe traditional use, not proven treatment.

• Difficult or painful urination (dysuria)
• Retention of urine
• Urinary incontinence and leakage on effort
• Frequent and urgent urination
• Constipation and difficult defecation
• Diarrhoea
• Low back and sacral pain
• Irregular menstruation
• Painful periods (dysmenorrhoea)
• Abnormal vaginal discharge (leucorrhoea)
• Lower abdominal distension and pain
• Pain and weakness running from the sacrum into the leg
• Anal and perineal pain
• Nocturnal emission and impotence, in the classical record

Why the urinary indications carry more weight here than at most points. Nearly every acupuncture point's indication list rests on the classical record alone. BL-33's urinary indications are unusual in having been tested: a multicentre randomised trial in 504 women, published in JAMA in 2017, used electroacupuncture at BL-33 and BL-35 against a sham and reported less urine leakage at six weeks, and a 2025 randomised trial in men after prostate surgery used BL-32, BL-33 and BL-34 with a similar result. Both are set out in full, with their limits, under research below. It remains the case that no trial isolates BL-33 from its neighbours.

Source: Sacred Lotus sources & references (existing indication record, reformatted here as a list from a bare comma-run); Deadman & Al-Khafaji, A Manual of Acupuncture, p. 294; Chinese Acupuncture & Moxibustion, p. 182; PMID 28655016; PMID 24079823; PMID 41026492 — each verified resolvable and checked for retraction status before citation; cross-referenced against multiple online sources.

Where is BL-33 Zhong Liao, and how is it used in practice?

Zhong Liao lies on the sacrum — the broad triangular bone at the base of the spine — directly over the third of the four paired openings in its back surface, below and slightly inside Ci Liao BL-32. Sacred Lotus sources & references hold two location records for it and they agree, one placing it medial and inferior to BL-32 at the third posterior sacral foramen and the other simply over that foramen. There is no discrepancy between them to flag. In our own words: it is the third hole down, counting from the top, on each side of the sacrum, and the four holes converge slightly as they descend, so BL-33 sits closer to the midline than BL-32 does.

The practical consequence is the single most useful thing to know about this point. Unlike almost every other point on the body, BL-33 has a target rather than a location: the question is not only where the needle enters but whether it enters the foramen or meets the bone beside it. That is why the sacral points, alone among acupuncture points, have generated a measurement literature about locating them — described under needling.

Which of the eight liao points does this site hold?

All four levels are held in Sacred Lotus sources & references, and the set was queried directly rather than recalled. Because each exists on both sides of the sacrum, the group of four is called the baliao — the "eight bone holes":

• BL-31 Shang Liao ("Upper Bone Hole") — over the first posterior sacral foramen.
• BL-32 Ci Liao ("Second Bone Hole") — over the second. The most used of the four, and the one reached for when a single level is chosen.
• BL-33 Zhong Liao ("Central Bone Hole") — over the third. This point.
• BL-34 Xia Liao ("Lower Bone Hole") — over the fourth, and the most anorectal of the four in its recorded indications.

Queried directly, all four carry the same Meeting Point of the Bladder and Gall Bladder channels classification and the same recorded needling range in our own data — which is itself informative: what distinguishes them is position, and therefore which sacral nerve they sit over, rather than any difference of category. They are frequently used as a block, with two or four needles a side. Hui Yang BL-35, half a cun either side of the midline level with the tip of the tailbone, sits below the whole group and is also held here; it is BL-33's partner in the largest trial in this territory.

Which points is BL-33 combined with?

• With BL-35 Hui Yang — the pairing used in the multicentre stress-incontinence trial described under research, and the reason that trial belongs on this page. Both are held here.
• With BL-32 above and BL-34 below — the ordinary sacral block. The 2025 post-prostatectomy trial used BL-32, BL-33 and BL-34 together; the animal study of slow-transit constipation used BL-33 with BL-34.
• With Pang Guang Shu BL-28 and Zhong Ji REN-03 — the Back-Shu and Front-Mu pair of the Bladder, both held here and queried. Adding the foramen point to that pair gives back, front and sacral access to the same organ. BL-28 lies at the second foramen level on the inner Bladder line; REN-03 lies on the lower abdomen directly over the bladder.
• With San Yin Jiao SP-06 — the Meeting Point of the Spleen, Liver and Kidney channels on the inner lower leg, held here, and the standard distal partner for anything urinary or gynaecological.
• With Shen Shu BL-23 — the Back-Shu point of the Kidneys, held here; the usual addition where a urinary presentation is read as weakness rather than heat.
• With Guan Yuan REN-04 and Qu Gu REN-02 — lower-abdominal points over the bladder, both held here, for front-and-back arrangements.
• With Cheng Shan BL-57 and Chang Qiang — for the anorectal indications, where the sacral points are combined with the points of the back of the leg and the tailbone. BL-57 is held here.
• With Wei Zhong BL-40 — behind the knee, held here, the Command point of the back and the standard distal partner for the local sacral action.

Source: Sacred Lotus sources & references (both location records; the complete four-level liao set and the BL-23, BL-28, BL-31, BL-32, BL-34, BL-35, BL-40, BL-57, REN-02, REN-03, REN-04 and SP-06 records queried directly); Deadman & Al-Khafaji, A Manual of Acupuncture, p. 294; Chinese Acupuncture & Moxibustion, p. 182; cross-referenced against multiple online sources.

Where is BL-33 located?

• A Manual of Acupuncture (p. 294): Over the third posterior sacral foramen.
• Chinese Acupuncture and Moxibustion (p. 182): On the sacrum, medial and inferior to Ciliao (BL-32), just at the third posterior sacral foramen.

How is BL-33 Zhong Liao needled, and what lies beneath it?

Stated plainly: BL-33 is a low-hazard point in the visceral sense — the sacrum lies between it and the pelvic cavity, so the organ-injury concern that governs the abdominal points does not apply. What governs this point instead is nerve, and whether the needle finds the foramen at all. Sacred Lotus sources & references record perpendicular insertion of 0.8 to 1.2 inches with moxibustion applicable — the same figure held here for BL-31, BL-32 and BL-34. Everything below is recorded as neutral reference data describing what the textbooks and the published anatomical studies document. It is not instruction, this page does not teach technique, and needling is carried out only by a qualified, licensed practitioner.

The angle and depth the texts record

• Sacred Lotus sources & references record perpendicular insertion of 0.8 to 1.2 inches, with moxibustion applicable.
• Deadman & Al-Khafaji (p. 294) and Chinese Acupuncture & Moxibustion (p. 182) record insertion at the third posterior sacral foramen in a comparable range, with the needle directed into the foramen.
• Moxibustion is recorded as applicable, and cupping and warming techniques are extremely common over the sacrum.

The foramen, measured — and these are BL-33's own figures

The most precise data on this point comes from a retrospective study of 100 prone pelvic CT scans, reconstructed in three dimensions, which measured the angle and depth at which a needle follows the axis of each sacral foramen. It reported every level separately, so the figures below are BL-33's own rather than borrowed from its better-known neighbour. The sacral foramina run obliquely — downward and inward — so a needle following the channel of a foramen is not perpendicular to the skin:

• Angle to the skin surface at BL-33: 58.25 ± 8.69 degrees. (For comparison, and labelled as theirs: BL-31 61.04 ± 12.15°, BL-32 57.57 ± 10.01°, BL-34 54.39 ± 10.94°.)
• Angle to the posterior midline at BL-33: 17.36 ± 7.90 degrees. (BL-31 24.54 ± 6.21°, BL-32 18.58 ± 7.76°, BL-34 30.73 ± 9.45°. BL-33 has the smallest midline angle of the four — the third foramen runs the most nearly straight back-to-front of the set.)
• Effective depth at BL-33: 33.96 ± 10.74 mm, defined as the thickness of the soft tissue over the sacrum plus half the depth of the sacrum. The four levels decrease steadily from top to bottom — BL-31 58.16 ± 12.43 mm, BL-32 44.57 ± 11.55 mm, BL-33 33.96, BL-34 31.13 ± 10.94 mm. Depth correlated positively with body mass index but not with sex or with weight alone (Zhen Ci Yan Jiu 2017;42(6):537–541, PMID 29318863).

Read that descending series carefully, because it contains the practical fact about this point. The effective depth at BL-33 is about three-quarters of the depth at BL-32 and barely more than half the depth at BL-31. The sacrum thins as it descends and the soft tissue over it thins with it. A figure taken from the second foramen and applied at the third would overshoot by roughly a centimetre — which is precisely why this record uses BL-33's own numbers and labels the others.

Those figures belong beside the textbook range, not instead of it. The 0.8–1.2 inch perpendicular figure in the general reference texts describes ordinary insertion; the CT measurements describe the deep oblique technique used in specialist practice, in which a longer needle follows the foramen toward the sacral nerve. They are different procedures and the numbers are not interchangeable. Chinese clinical writing on the deep baliao method states this openly — that the effect its practitioners claim depends on deep insertion through the foramen with long needles, that the angle must be adjusted to the shape of each individual foramen, and that descriptions of location and manipulation in both ancient and modern literature have been vague enough to restrict the technique's spread (Zhongguo Zhen Jiu 2014;34(3):285–288, PMID 24843976).

Whether the point is actually found has been measured — twice, and both studies name BL-33

• Ultrasound localisation. A 2025 randomised study of 52 patients with urinary retention after spinal cord injury addressed the question directly: both groups received electroacupuncture at Ci Liao BL-32 and Zhong Liao BL-33, but in one the foramina were located first by ultrasound. The ultrasound-guided group did significantly better on lower urinary tract symptom scores, urination frequency, urine volumes and residual urine, and the authors reported a correlation between how accurately the points were located and the size of the effect (Zhen Ci Yan Jiu 2025;50(10):1169–1177, PMID 41116990). Single centre, modest size, and the ultrasound arm cannot be blinded — but the finding bears directly on the anatomical particularity above.
• Depth, tested as the variable. A single-blind randomised trial allocated 120 men with benign prostatic hyperplasia to electroacupuncture at Ci Liao BL-32 and Zhong Liao BL-33 at deep versus shallow insertion, three times a week for four weeks. Both depths improved symptoms; the deep-insertion group did significantly better on the International Prostate Symptom Score, quality-of-life score and effective rate (P < 0.05), while maximum urine flow rate and post-void residual did not differ significantly between the groups (Yuan H et al., Evid Based Complement Alternat Med 2019;2019:1439141, PMID 31915442, PMC6930788). This is an unusual and useful study: it treats depth at BL-33 as the intervention rather than as a footnote. Its limits are single-centre design, subjective primary outcomes, and the near-impossibility of blinding a patient to how deep a needle went.

What the recorded hazards at this point are, and are not

• The relevant structure is nerve, not organ. The third posterior sacral foramen transmits the posterior ramus of the third sacral spinal nerve. The sacrum separates the point from the pelvic cavity, so the visceral concern that governs the abdominal points is not the governing concern here; the phenomena recorded at the sacral points are electric or radiating sensation into the buttock, perineum or leg, and local soreness. In the deep baliao technique that sensation is described as the response sought, which is a description of a nerve responding.
• No single agreed safe depth exists for any point. Two systematic reviews of the safe-depth literature — 33 studies in the first, 47 in the second, by MRI, CT, ultrasound and cadaveric dissection — both concluded that measured depths for the same point vary greatly between subject groups and measuring methods, and that "safe depth" has never been standardised (J Altern Complement Med 2011;17(3):199–206, PMID 21417806; Evid Based Complement Alternat Med 2013;2013:740508, PMID 23935678).
• Where the serious events of acupuncture actually cluster. Systematic reviews of the Chinese case literature place them over the chest, the abdomen, the neck and the orbit, attributed chiefly to improper technique (He W et al., PMID 22967282). The sacrum is not among them.
• Risk in proportion. A meta-analysis of prospective clinical studies estimated serious adverse events at approximately 1 per 10,000 patients and about 8 per million treatments, with roughly half of all recorded events being bleeding, pain or flare at the needle site (Bäumler P et al., BMJ Open 2021;11:e045961, PMID 34489268). In the 504-woman electroacupuncture trial that used this point, treatment-related adverse events occurred in 1.6 per cent of the electroacupuncture group and 2.0 per cent of the sham group, and all were classified as mild (PMID 28655016).
• Honest limit. No published case report naming BL-33 specifically in connection with an injury was located, and none is asserted.

Source: Sacred Lotus sources & references (existing needling record and both location records; the BL-31, BL-32 and BL-34 records queried directly); Deadman & Al-Khafaji, A Manual of Acupuncture, p. 294; Chinese Acupuncture & Moxibustion, p. 182; PMID 29318863 (BL-33's own figures used; BL-31, BL-32 and BL-34 values labelled as theirs); PMID 24843976; PMID 41116990; PMID 31915442 (PMC6930788); PMID 28655016; PMID 21417806; PMID 23935678; PMID 22967282; PMID 34489268 — each verified resolvable and checked for retraction status before citation.

What does the name Zhong Liao mean?

Zhong Liao means "Central Bone Hole" — zhong, central or middle, and liao, a hole or crevice in bone. Sacred Lotus sources & references translate it exactly that way. Like its three companions the name is a plain anatomical instruction rather than a metaphor: it is the middle one of the holes. The four are named purely by position — Shang Liao BL-31 (Upper), Ci Liao BL-32 (Second), Zhong Liao BL-33 (Central) and Xia Liao BL-34 (Lower) — and because each exists on both sides of the sacrum the group is collectively the baliao, the "eight bone holes". All four are held in this library.

One naming oddity is worth flagging, since it makes the set look inconsistent. Three of the four names are positional in the ordinary way — upper, central, lower. The second is not: ci means "next in order" rather than naming a place in the sequence. So the run reads upper, next, central, lower, which is not how a modern person would number four things. The likeliest reading is that the names accumulated rather than being designed as a set, and it is a small reminder that this vocabulary is descriptive and historical rather than systematic.

The character liao is not peculiar to the sacrum. It appears across the body wherever a point is named for a hollow or depression in bone — our own records hold, among others, ST-03 Ju Liao (Great Bone Hole) on the face, SI-18 Quan Liao (Cheek Bone Hole), LI-12 Zhou Liao (Elbow Bone Hole) on the arm and GB-29 Ju Liao (Squatting Bone Hole) at the hip. Two of those, ST-03 and GB-29, share the pinyin Ju Liao while being different points with different characters; our GB-29 record sets that collision out in full.

Why does the third foramen in particular matter?

Because of what emerges from it. The third sacral nerve is a principal contributor to the nerve supply of the bladder and the pelvic floor, and the whole modern career of BL-33 follows from that. It is the point behind sacral-nerve-directed acupuncture for incontinence, retention and pelvic floor dysfunction, and it is the reason the largest trials in this territory chose this level rather than the more famous BL-32 above it. In an ordinary channel point the relationship between position and effect is theoretical; here it is anatomical and testable, which is why the evidence below is better than most acupuncture points can claim.

Is there research on BL-33 Zhong Liao?

Yes — genuinely, and more than most acupuncture points have. BL-33 is named in the protocol of the largest sham-controlled acupuncture trial for stress urinary incontinence ever conducted, in a 2025 randomised trial after prostate surgery, in a randomised trial that treated needling depth at this point as the intervention, and in an ultrasound-localisation trial. What it does not have is a trial of BL-33 on its own; every one of those studies pairs it with a neighbour, and that is stated in each case.

• The stress-incontinence trial that is about this point — and is routinely mis-cited. A multicentre randomised trial at 12 hospitals in China enrolled 504 women with pure stress urinary incontinence, assigning them 1:1 to 18 sessions over six weeks of electroacupuncture involving the lumbosacral region or of sham electroacupuncture with non-penetrating needles at sham locations. At week 6 the electroacupuncture group had a greater decrease in mean urine leakage on the one-hour pad test (−9.9 g versus −2.6 g; mean difference 7.4 g, 95% CI 4.8 to 10.0, P < .001), with smaller between-group differences in 72-hour incontinence episodes that persisted at weeks 27–30. Treatment-related adverse events were 1.6 per cent versus 2.0 per cent and all were mild (Liu Z et al., JAMA 2017;317(24):2493–2501, PMID 28655016). The trial report itself does not name the points in its abstract, and the trial is very widely cited as evidence for "the sacral points" or specifically for BL-32. It is not. The published protocol names them: the treatment group received electroacupuncture with deep needling at BL33 and BL35 (Trials 2013;14:315, PMID 24079823, PMC3850726), verified at source. Our own BL-32 record states the same correction from its side, so that a reader does not carry the result across. The result belongs here — and so do its limits: it is a trial of two points used together, not of BL-33 alone; the comparator was a non-penetrating sham, which controls for expectation imperfectly; the primary outcome was measured at six weeks; and a 7.4 g difference on a one-hour pad test is a real but modest quantity.
• A 2025 randomised trial after prostate surgery, also naming this point. A single-centre, single-blinded randomised trial enrolled 110 men with urinary incontinence four to six weeks after robot-assisted radical prostatectomy, assigning them to electroacupuncture at bilateral Ci Liao BL-32, Zhong Liao BL-33 and Xia Liao BL-34 — sparse-dense wave, alternating 2 and 15 Hz, 30 minutes, three times a week for six weeks — or to sham stimulation with non-puncturing flat-tip needles at offset points. At week 6 the electroacupuncture group achieved a higher urinary continence rate on intention-to-treat analysis (24 of 55, 43.6 per cent, versus 12 of 55, 21.8 per cent; relative risk 2.00, 95% CI 1.12–3.59; P = .02) (Niu J et al., JAMA Netw Open 2025;8(9):e2534491, PMID 41026492, PMC12485641). Limits: single centre, three points used together, and a six-week primary endpoint in a condition that often improves on its own over months.
• Depth at BL-33 treated as the intervention. The 120-patient benign prostatic hyperplasia trial described under needling randomised deep against shallow electroacupuncture at BL-32 and BL-33 and found the deep arm better on symptom and quality-of-life scores but not on flow rate or residual volume (PMID 31915442, PMC6930788).
• Localisation at BL-33 treated as the intervention. The ultrasound-guided trial in 52 patients with urinary retention after spinal cord injury, also described under needling, needled BL-32 and BL-33 in both arms and differed only in whether the foramina were located by ultrasound first (PMID 41116990).
• Mechanism, in animals, at BL-33 by name. Thirty-two rats with loperamide-induced slow-transit constipation received electroacupuncture (2 Hz/15 Hz, 30 minutes daily for 14 days) at bilateral "Zhongliao" BL-33 and "Xialiao" BL-34. Compared with the model group, the electroacupuncture group showed shorter time to first black stool, higher faecal water content, increased colonic 5-HT and expression of the 5-HT4 receptor, tryptophan hydroxylase 1 and the serotonin transporter, raised serum substance P and lowered vasoactive intestinal peptide, and increased faecal acetic and butyric acid; the authors concluded that electroacupuncture at these points promotes intestinal motility through multiple links of the serotonin signalling system (Ma JZ et al., Zhen Ci Yan Jiu 2022;47(12):1101–1106, PMID 36571226). Limits: 32 rats, eight per group, a drug-induced model, and two points used together. It is included because it is a mechanistic measurement made at this point by name and because it addresses the bowel half of the point's recorded action, which the human trials do not.
• The wider baliao literature. A survey of the clinical literature on the eight liao points as a group analysed their indication patterns across the published body of work (Zhongguo Zhen Jiu 2019;39(1), PMID 30672264). It describes the group, not BL-33 individually.
• What is not here. There is no randomised trial of BL-33 alone, no sham-controlled study isolating it from BL-32, BL-34 or BL-35, and no trial at all of its menstrual or discharge indications. Those rest entirely on the classical record and the reference literature. The fair summary is that BL-33 has better human evidence than almost any other single acupuncture point in the urinary field — and that none of it separates the point from its neighbours.

How does Zhong Liao compare with the points around it?

BL-31, BL-32 and BL-34 are the same point at the first, second and fourth foramina; the set descends, and with it the emphasis shifts from the higher pelvic organs toward the anus and perineum, BL-34 being the lowest and most anorectal. BL-32 Ci Liao is the best known of the four and the usual choice when a single level is picked, which is exactly why the trial evidence that actually belongs to BL-33 so often ends up attached to it. BL-35 Hui Yang sits below the whole group beside the tip of the tailbone and is BL-33's partner in the largest trial. Pang Guang Shu BL-28, the Back-Shu point of the Bladder, lies one and a half cun from the midline at the second foramen level — the same channel, the inner line, and classified as a Back-Shu point where BL-33 is not. Zhong Lu Shu BL-29 lies on that inner line at BL-33's own level, and Bai Huan Shu BL-30 at the fourth. Zhi Bian BL-54 lies out on the buttock, three cun from the sacro-coccygeal hiatus. All of these are held in this library and all were queried.

Source: Sacred Lotus sources & references (name, translation, channel, the Meeting Point classification, the complete four-level liao set and the BL-28, BL-29, BL-30, BL-31, BL-32, BL-34, BL-35, BL-54, GB-29, ST-03 and SI-18 records queried directly); Deadman & Al-Khafaji, A Manual of Acupuncture, p. 294; Chinese Acupuncture & Moxibustion, p. 182; PMID 28655016; PMID 24079823 (PMC3850726); PMID 41026492 (PMC12485641); PMID 31915442 (PMC6930788); PMID 41116990; PMID 36571226; PMID 30672264; PMID 29318863 — each verified resolvable and checked for retraction status before citation; cross-referenced against multiple online sources.