Long-Term Clinical Outcomes for Bone-Anchored Hearing Implants: 3-Year Comparison Between Minimally Invasive Ponto Surgery and the Linear Incision Technique With Tissue Preservation

Abstract

Objective

To compare the 3-year outcomes of the modified minimally invasive Ponto surgery (m-MIPS) to both the original MIPS (o-MIPS) and linear incision technique with soft tissue preservation (LIT-TP) for inserting bone-anchored hearing implants (BAHIs).

Study design

Prospective study with three patient groups: m-MIPS, o-MIPS, and LIT-TP.

Setting

Tertiary referral center.

Patients

In the m-MIPS group, 24 patients with 25 implants were enrolled. The o-MIPS and LIT-TP groups included 25 patients (25 implants) each.

Interventions

The patients underwent BAHI surgery using m-MIPS, o-MIPS, or LIT-TP.

Main outcome measures

Implant survival and implant stability were compared between all groups. Soft tissue status, skin sensibility, subjective numbness, and hearing-related quality of life (HRQoL) were compared between m-MIPS and LIT-TP and o-MIPS and LIT-TP, respectively.

Results

Implant survival was comparable between m-MIPS and LIT-TP (96 versus 100%), with o-MIPS showing nonsignificant lower survival (88%). Both MIPS groups exhibited fewer (adverse) skin reactions, better skin sensibility, and less subjective numbness than LIT-TP throughout visits. At 3 years, soft tissue status, sensibility, and numbness were comparable between groups. Device use was consistent among groups (83–86% daily users). All groups demonstrated significant improvement in HRQoL post-surgery based on GBI, GHSI, and APHAB total scores.

Conclusions

Compared with LIT-TP, m-MIPS showed comparable long-term implant survival, fewer (adverse) skin reactions, and earlier sensibility and numbness recovery. M-MIPS resulted in favorable clinical and QoL outcomes with low intra- and postoperative complication rates until 3 years after surgery. It is therefore considered a safe technique for BAHI insertion. Moreover, with a shorter surgery time and the ability to operate under local anesthesia in a controlled outpatient setting, m-MIPS appears to be a more efficient alternative to LIT-TP.

INTRODUCTION

The linear incision technique with soft tissue preservation (LIT-TP) is the gold standard for bone-anchored hearing implants (BAHIs) insertion due to its favorable postoperative outcomes (1–4). In 2014, minimally invasive Ponto surgery (MIPS) aimed to further reduce complications, but there were reports of higher implant loss rates as compared with LIT-TP (5–9). Proposed factors contributing to this include the presence of interposed periosteum, incorrect angulation of the drill/implant, and inadequate bone cooling, causing thermal necrosis and impaired osseointegration (6,7,9,10). To address this, drills were modified to enhance efficiency and osteotomy preparation (11). In addition, the three-step drilling sequence stipulated in the surgical manual was utilized. The 6-month outcomes of modified MIPS drills (m-MIPS) were published previously and were promising, with low intra- and postoperative complication rates (12). Therefore, m-MIPS seemed an excellent alternative to the LIT-TP, although long-term clinical- and quality-of-life (QoL) outcomes of m-MIPS were still lacking. This study, a continuation of two previously published 6-month follow-up studies (9,12), investigated the 3-year clinical outcomes and QoL after m-MIPS compared with the LIT-TP and o-MIPS (i.e., MIPS with the original drill design) compared with the LIT-TP.

MATERIALS AND METHODS

Ethical Considerations

This study was approved by the local ethical committee and performed according to the guidelines for Good Clinical Practice, ISO14155:2011, and the ethical principles stated by the Declaration of Helsinki (13). All patients provided written informed consent.

Study Population

This study comprised three patient groups. Adult patients were prospectively included and implanted with a BAHI in previously conducted clinical trials using 1) m-MIPS (9,12), 2) o-MIPS (9), or 3) LIT-TP (1,2). Study design, inclusion and exclusion criteria, outcome measures, and follow-up visits were identical among groups. External monitoring ensured quality control.

Surgical Techniques and Follow-Up

All patients underwent single-stage surgery with the Wide Ponto implant (diameter 4.5 mm, length 4.0 mm; Oticon Medical AB, Askim, Sweden) and abutment (6, 9, or 12 mm). Experienced ENT surgeons (E.M. and M.H.) performed the operations. Abutment length was chosen based on the implant site's skin thickness using a thin needle and a ruler. The ruler provides a comparative guide between skin thickness and Ponto abutment length: 6 mm abutment for 0.5–3 mm skin thickness, 9 mm for 3–6 mm, 12 mm for 6–9 mm, and 14 mm for 9–12 mm.

In m-MIPS (Fig. 1), the technique involves a circular incision using a 5-mm biopsy punch, followed by periosteum removal with a raspatorium. Subsequently, a cannula is inserted perpendicular to the temporal bone, facilitating three-step drilling with continuous saline irrigation and cannula flushing between steps. The drilling sequence includes 1) creating a guide hole using the cannula guide drill with spacer, 2) deepening the hole for a 4-mm implant using the same drill without spacer, 3) widening the hole with the cannula widening drill. After cannula removal, the implant with premounted abutment is inserted, and an insertion indicator ensures full implant insertion. Manual tightening is performed if necessary.

FIG. 1.

Modified minimally invasive Ponto surgery (MIPS)*: A circular incision is created using a 5-mm biopsy punch (1). The cannula is inserted (2). Three-step drilling is performed (3, 4). The cannula is removed, and the implant is inserted (5). A healing cap with dressing is attached to the abutment (6). *Published with permission from Oticon Medical AB.

In o-MIPS, the procedure differed from m-MIPS by the use of previous-generation drill designs and two-step drilling instead of three-step drilling.

In the LIT-TP, a linear incision was made to the periosteum (14), followed by periosteum removal and a two-step drilling sequence, aligning with our clinic's standard practice. The guide hole is created to total depth in one step before the hole is widened. After implant-abutment insertion, skin closure is done with two resorbable sutures, and a 5-mm punch hole allows abutment penetration (15).

Aftercare was consistent across all groups. Directly post-surgery, a healing cap and antibiotic-corticosteroid dressing were applied around the abutment. After 7 days, the healing cap was removed, and topical Terra-Cortril ointment was prescribed for twice-daily application around the abutment for 2 weeks. Additional follow-up visits were scheduled at 21 days (including sound processor fitting), 12 weeks, and 6, 12, 24, and 36 months post-surgery.

Outcome Measures

Implant survival and stability were compared between m-MIPS, LIT-TP, and o-MIPS. Additionally, soft tissue reactions, skin sensibility around the abutment, subjective numbness, device use, and hearing-related quality of life (HRQoL) were compared between m-MIPS versus LIT-TP and o-MIPS versus LIT-TP. Unplanned visits, the need for revision surgery, and adverse events were documented.

To gauge implant stability, the Implant Stability Quotient (ISQ) was determined through resonance frequency analysis directly post-implant insertion and at follow-up visits (1,2,16), using the Osstell ISQ device (Osstell AB, Göteborg, Sweden) and a SmartPeg (type 55). Perpendicular measurements were performed and reported as the highest and lowest scores, ranging from 0 to 100, with higher values indicating a more rigid implant–bone interface. Soft tissue tolerability was assessed using the Holgers classification (17) and the IPS scale (18). Because the IPS scale was introduced after the implantation of the LIT-TP group, the IPS score in this group was retrospectively assessed using the notes available in the patient record. Adverse skin reactions were considered for Holgers ≥2 or an IPS score indicating treatment. The presence of skin dehiscence, including size (in mm), was reported for MIPS groups. Skin sensibility around the abutment was measured at standardized locations using gnostic and vital sensibility tests (1,2,9). Gnostic sensibility used a soft end, whereas vital sensibility was assessed by using the sharp end of a broken cotton swab. Correct response percentages were compared between MIPS and LIT-TP. Subjective numbness was assessed using a visual analog scale (VAS) ranging from 0 (no numbness) to 10 (complete numbness). HRQoL was evaluated with the Glasgow Benefit Inventory (GBI) at 12 weeks and 12 months post-surgery, the Glasgow Health Status Inventory (GHSI), and the Abbreviated Profile of Hearing Aid Benefit (APHAB), both at screening and at 6 and 36 months post-surgery.

The GBI and GHSI comprise 18 questions scored on a five-point scale, resulting in a total score and three subscores (19,20). The GBI evaluates the patients' perceived benefit from an intervention, with scores ranging from −100 (maximal adverse/negative effect), 0 (no effect/benefit), to +100 (maximal positive effect). The GHSI assesses the impact of a health problem on the QoL of an individual at the time of questionnaire completion, with scores ranging from 0 to 100, where higher scores indicate better QoL.

The APHAB, a 24-item inventory, is scored in four 6-item subscales on communication abilities or sound perception in daily life situations. All items are scored on a seven-point scale indicating the frequency of problems experienced, ranging from 1 to 99%, with higher scores indicating more problems. Hence, a negative change indicates improvement.

Statistical Analysis

Data management and statistical analyses were performed by independent external data managers and biostatisticians (Statistika Konsultgruppen, Göteborg, Sweden) and executed according to predefined statistical analysis plans. Nonparametric tests were employed, including Fisher nonparametric permutation test for numbness variables, Mann–Whitney U test for continuous variables, Mantel–Haenzsel chi-square test for ordered categorical variables, chi-square test for nonordered categorical variables, and Fisher exact test for dichotomous variables. Changes over time were assessed using the Wilcoxon signed-rank test for continuous variables and the Sign test for dichotomous and ordered categorical variables. The log-rank survival test compared implant survival between groups. All collected data up to the withdrawal point were included in case of premature withdrawal. No imputations for missing data were made. For bilaterally implanted patients, demographics and questionnaire results were handled on a patient level, and implant-related variables on an implant level. Patients included in more than one group were treated as separate subjects. All tests, performed by using SAS v9.4 (Cary, NC), were two-tailed and conducted at a 0.05 significance level.

RESULTS

Patient Population

In the m-MIPS group, 24 patients with 25 implants were enrolled from September 2018 to June 2019. The o-MIPS and LIT-TP groups included 25 patients (25 implants) each, recruited between June and December 2017 and February and August 2014, respectively (1,9). Two patients underwent sequential bilateral implantation, the first implant inserted using o-MIPS and the second using m-MIPS. There were no differences in characteristics between groups (see Table 1). Of the 74 patients initially included, 9 did not complete the 3-year follow-up due to reasons such as implant loss (1 in m-MIPS and 3 in o-MIPS) and abutment removal (1 in o-MIPS and 2 in LIT-TP). One patient in the m-MIPS group passed away, and one patient in the LIT-TP group was lost to follow-up for unknown reasons.

TABLE 1.

Patient and surgical characteristics for the test group and two control groups

Variablea	Modified MIPS Test Group	LIT-TP Control Group 1	Original MIPS Control Group 2
Patient variables	n = 24	n = 25	n = 25
Gender, n (%)
Male	9 (38)	15 (60)	9 (36)
Female	15 (63)	10 (40)	16 (64)
Age, mean (SD), yr	53 (14)	52 (13)	60 (13)
Ethnicity, n (%)
Caucasian	23 (96)	25 (100)	25 (100)
Hispanic	1 (4)	0 (0)	0 (0)
Smoking, n (%)	6 (25)	4 (16)	5 (20)
Relevant diseases, n (%)
Diabetes mellitusb	3 (13)	0 (0)	3 (12)
Skin disease	1 (4)	1 (4)	0 (0)
Chronic steroid use	0 (0)	1 (4)	0 (0)
Indication, n (%)
Acquired conductive/mixed	17 (71)	21 (84)	20 (80)
Congenital conductive	0 (0)	1 (4)	1 (4)
Single-sided deafness	7 (29)	3 (12)	4 (16)
Bilateral implantation, n (%)	1 (4)	0 (0)	0 (0)
Abutment length, n (%)
6 mm	1 (4)	0 (0)	1 (4)
9 mm	14 (56)	17 (68)	14 (56)
12 mm	10 (40)	8 (32)	10 (40)

^a Variables were compared between modified MIPS and LIT-TP, and between modified MIPS and original MIPS.

^b Type 2 diabetes mellitus with stable blood glucose levels and treatment with dietary restrictions and/or oral diabetes medication.

LIT-TP indicates linear incision technique with soft tissue preservation; MIPS, minimally invasive Ponto surgery; SD, standard deviation.

Implant Survival and Stability

In the m-MIPS group, one implant was lost within 10 weeks postoperatively, preceded by complaints of pain (VAS 7). The o-MIPS group had three losses (two spontaneous, one after trauma) within the same 10-week time frame, whereas the LIT-TP group experienced no implant losses. No additional implant losses occurred between 10 weeks and the 3-year visit for the entire study population. There were no significant differences in implant survival between m-MIPS and o-MIPS (p = 0.297), between m-MIPS and LIT-TP (p = 0.317), or between o-MIPS and LIT-TP (p = 0.077). In m-MIPS and o-MIPS, three patients and one patient, respectively, changed to a shorter abutment because the abutment protruded too much. Additionally, in m-MIPS and o-MIPS, one abutment and three abutments, respectively, were removed due to patient dissatisfaction or progressive sensorineural hearing loss requiring planned cochlear implantation.

The mean 0- to 36-month areas under the curve (AUCs) of the ISQ-low and -high were significantly higher for LIT-TP than m-MIPS and o-MIPS (only for 12-mm abutments). However, the mean 0- to 36-month AUCs of the ISQ-low and -high were comparable between m-MIPS and o-MIPS (see Table 1). Implant stability significantly improved over time in all groups, leveling out after 12 months. Figure 2 illustrates the ISQ-low and -high over time for m-MIPS and LIT-TP.

FIG. 2.

ISQ values for the m-MIPS group (green), o-MIPS group (orange), and LIT-TP group (purple) at surgery and at 3, 6, 12, 24, and 36 months after surgery for the 9-mm (A) and the 12-mm abutment (B). The mean is marked by a cross, and the median by a horizontal line. The boxplots represent the interquartile range, and the dots represent outliers. LIT-TP, linear incision technique with soft tissue preservation; m-MIPS, modified minimally invasive Ponto surgery; o-MIPS, original minimally invasive Ponto surgery.

Soft Tissue Status

Figure 3 presents Holgers and IPS scores across visits. Holgers scores differed significantly between m-MIPS and LIT-TP at 6 months, with worse scores for LIT-TP (p = 0.049). Maximum Holgers scores throughout visits were significantly higher for LIT-TP than m-MIPS and o-MIPS (see Table 2). The highest reported score was Holgers 2 in m-MIPS and o-MIPS (8 and 12%, respectively) and Holgers 3 (12%) in LIT-TP. Moreover, adverse Holgers scores were significantly more prevalent in LIT-TP patients than in m-MIPS patients (36 versus 8%, p = 0.037).

FIG. 3.

Soft tissue reactions across all visits according to the Holgers scale (A) and the IPS scale (B) for the m-MIPS group (green), o-MIPS group (orange), and LIT-TP group (purple). Holgers ≥2 and IPS scores indicating treatment were considered to be adverse skin reactions. LIT-TP, linear incision technique with soft tissue preservation; m-MIPS, modified minimally invasive Ponto surgery; o-MIPS, original minimally invasive Ponto surgery.

TABLE 2.

Outcome measures compared between test and control groups

Outcome Measure	m-MIPS	o-MIPS	LIT-TP	p Value m-MIPS versus LIT-TP	p Value o-MIPS versus LIT-TP	p Value m-MIPS versus o-MIPS
Implant loss 0–36 mo, n (%)	n = 25	n = 25	n = 25
Implant loss	1 (4.0)	3 (12.0)	0 (0.0)	0.317	0.077	0.297
Mean AUC ISQ 0–36 mo (SD)	n = 25	n = 25	n = 25
6-mm abutment, ISQ-lowa	68.5	68.6
6-mm abutment, ISQ-higha	69.7	69.4
9-mm abutment, ISQ-low	58.3 (3.5)	60.5 (4.5)	61.8 (2.7)	0.010	0.65	0.14
9-mm abutment, ISQ-high	59.8 (3.1)	62.3 (4.1)	63.0 (2.8)	0.005	0.83	0.085
12-mm abutment, ISQ-low	51.6 (3.3)	51.1 (4.3)	57.0 (3.5)	0.019	0.011	0.57
12-mm abutment, ISQ-high	53.1 (3.8)	53.1 (3.9)	58.5 (3.4)	0.009	0.019	0.85
Maximum Holgers 0–36 mo, n (%)b	n = 25	n = 25	n = 25
0	16 (64)	15 (60)	7 (28)
1	7 (28)	7 (28)	9 (36)
2	2 (8)	3 (12)	6 (24)
3	0 (0)	0 (0)	3 (12)
4	0 (0)	0 (0)	0 (0)	0.004	0.013
Maximum I, P, S scores 0–36 mo, n (%)b	n = 25	n = 25	n = 25
I score (inflammation)
0	5 (20)	5 (20)	8 (32)
1	16 (64)	13 (52)	9 (36)
2	4 (16)	7 (28)	3 (12)
3	0 (0)	0 (0)	5 (20)
4	0 (0)	0 (0)	0 (0)	0.44	0.76
P score (Pain)
0	19 (76)	15 (60)	11 (44)
1	5 (20)	8 (32)	13 (52)
2	1 (4)	2 (8)	1 (4)	0.083	0.65
S score (Skin height)
0	22 (88)	21 (84)	10 (40)
1	3 (12)	4 (16)	15 (60)
2	0 (0)	0 (0)	0 (0)	<0.001	0.003
Sensibility at 36 mo, mean % (SD)	n = 22	n = 19	n = 22
Total sensibility	100 (0.0)	100 (0.0)	99.6 (1.8)	1.00	1.00
Gnostic sensibility	100 (0.0)	100 (0.0)	99.2 (3.6)	1.00	1.00
Vital sensibility	100 (0.0) (0.0)	100 (0.0)	100 (0.0) (3.(3.3)	1.00	1.00
Subjective numbness at 36 mo, mean (SD)	n = 22	n = 20	n = 22
VAS	0.0 (0.0) 0.0 (0.0)	0.0 (0.0)	0.0 (0.0)	1.00	1.00
Sound processor use at 36 mo	n = 23	n = 20	n = 22
Daily users, n (%)	19 (83)	15 (75)	19 (86)	1.00	0.59
GBI at 12 mo, mean (SD)	n = 22	n = 21	n = 24
Total	31 (19)	26 (19)	32 (22)	0.72
General subscale	45 (25)	36 (23)	43 (29)	0.70
Social subscale	8 (22)	5 (16)	13 (22)	0.69
Physical subscale	−3 (17)	4 (17)	5 (19)	0.28
Change in GHSI 0–36 mo, mean (SD)c	n = 6	n = 6	n = 6
Total score	9.4 (9.9)	9.3 (7.7)	13.7 (24.3)	1.00	0.94
General score	12.2 (12.6)	12.5 (11.0)	18.4 (34.8)	1.00	0.94
Social score	−2.8 (13.6)	−1.4 (17.8)	−1.7 (9.1)	0.71	0.85
Physical score	9.7 (17.8)	7.0 (12.2)	6.9 (13.4)	0.62	0.81
Change in GHSI 0–36 mo, mean (SD)d	n = 15	n = 13	n = 20
Total score	9.8 (7.7)	7.2 (9.5)	17.9 (12.8)	0.034	0.003
General score	14.6 (9.3)	5.9 (16.3)	25.2 (18.3)	0.051	0.002
Social score	−0.6 (9.7)	7.8 (12.5)	0.2 (11.2)	0.74	0.055
Physical score	1.1 (18.6)	3.7 (7.9)	6.7 (13.7)	0.47	0.69
Change in APHAB 0–36 mo, mean (SD)c	n = 5	n = 5–6	n = 6
Global score	−13.0 (13.3)	−19.2 (18.2)	−16.0 (21.4)	0.93	0.69
Ease of communication	1.0 (11.1)	−7.4 (18.5)	−22.9 (27.6)	0.12	0.30
Background noise	−26.2 (19.8)	−26.3 (13.5)	−8.6 (24.9)	0.17	0.17
Reverberation	−9.4 (16.6)	−29.8 (23.5)	−16.4 (22.7)	0.52	0.24
Aversiveness	3.7 (20.1)	−1.6 (19.1)	0.1 (19.0)	0.78	1.00
Change in APHAB 0–36 mo, mean (SD)d	n = 9–15	n = 11–13	n = 18–19
Global score	−20.1 (19.8)	−24.4 (17.0)	−29.0 (26.4)	0.21	0.36
Ease of communication	−12.4 (21.5)	−18.5 (15.5)	−23.4 (34.2)	0.11	0.36
Background noise	−33.0 (16.6)	−31.6 (23.8)	−32.8 (28.2)	0.98	0.92
Reverberation	−22.5 (17.5)	−30.2 (18.7)	−30.5 (28.4)	0.35	0.76
Aversiveness	14.0 (29.4)	6.7 (21.3)	7.6 (29.9)	0.51	0.82

Results are presented for the ITT population.

^a Only two 6-mm abutments were used, one in the modified MIPS group and one in the original MIPS group.

^b All visits including unplanned visits.

^c Change in score between baseline with hearing aid to 36 months with hearing aid.

^d Change in score between baseline without hearing aid to 36 months with hearing aid.

AUC indicates area under the curve; ISQ, implant stability quotient; LIT-TP, linear incision technique with soft tissue preservation; MIPS, minimally invasive Ponto surgery; m-MIPS, modified MIPS; o-MIPS, original MIPS; SD, standard deviation; VAS, visual analog scale.

According to the IPS, the distribution of total scores between both MIPS groups and LIT-TP at 7 and 21 days post-surgery differed significantly in favor of LIT-TP (p < 0.005), whereas at 12 weeks and 36 months, it differed significantly in favor of m-MIPS (p < 0.05). Specifically, a statistically higher Inflammation (I) score was observed for both MIPS at 7 and 21 days due to wound dehiscence, i.e., a small gap between the skin and the abutment. In contrast, a statistically higher I score was observed for LIT-TP compared with m-MIPS at 6 months. The Skin height (S) score differed significantly in favor of m-MIPS at 12 weeks and 24 months, whereas the Pain (P) score did not differ between groups at any time point. Throughout visits, the maximum I and P scores showed no significant difference between m-MIPS and LIT-TP, whereas the maximum S score was significantly higher for the LIT-TP (p < 0.001; see Table 2). Adverse IPS scores were nonsignificantly more prevalent in the LIT-TP patients than in m-MIPS patients (48 vs 28%, p = 0.24).

All adverse skin reactions were successfully treated with antibiotic ointment, and no patients required revision surgery.

Skin Sensibility and Subjective Numbness

Total (gnostic and vital) sensibility across visits is presented in Figure 4. At 21 days after surgery, sensibility was significantly better for m-MIPS and o-MIPS compared with the LIT-TP (100 and 98% versus 93%, p < 0.001 and p = 0.017). Overall, both MIPS groups exhibited consistently favorable total sensibility across all visits. However, the change in total sensibility from baseline to 36 months did not show significant differences between the groups. Generally, total sensibility returned to baseline levels at 12 weeks for m-MIPS, at 21 days for o-MIPS, and at 6 months for LIT-TP, though it should be noted that skin sensibility was not measured in LIT-TP at the 12-week visit. Subjective numbness was significantly superior for m-MIPS and o-MIPS at 21 days (VAS 0.2 ± 1.0 and 0.0 ± 0.0 versus 2.2 ± 2.7, p < 0.001) and 12 months (VAS 0.0 ± 0.0 and 0.0 ± 0.0 versus 1.6 ± 2.3, p < 0.001), whereas it was comparable between groups at 36 months (see Table 2).

FIG. 4.

Total skin sensibility around the abutment at screening, at 21 days, and at 6, 12, and 36 months after surgery using the m-MIPS group (green), o-MIPS group (orange), and LIT-TP (purple). LIT-TP, linear incision technique with soft tissue preservation; m-MIPS, modified minimally invasive Ponto surgery; o-MIPS, original minimally invasive Ponto surgery.

Device Use

Device use is presented in Table 2. Patients had their sound processor fitted on the 21-day follow-up (19–32 days post-surgery). At 36 months, 15 patients in the o-MIPS group and 19 patients in both the m-MIPS and LIT-TP groups used their sound processor daily, with a median use of 16 hours (range: 8–16, 12–16, and 2–17 h, respectively). Six patients, all MIPS, were not using the sound processor. Reasons for non-usage included a preference for conventional devices, perceived lack of benefit from the second device (sequential bilateral implantation), (temporary) device malfunction, and (unexpected) decreased sensorineural hearing loss warranting cochlear implantation. For patients not using the sound processor daily, median use was 2.8 days per week.

Hearing-Related Quality of Life

Table 2 shows all groups' GBI, GHSI, and APHAB outcomes, whereas the raw GHSI and APHAB data can be found in the supplementary material (Table S1, http://links.lww.com/MAO/C11). At 12 months, the GBI total score revealed a substantial and clinically significant improvement post-intervention, with 95.6% of all patients experiencing an enhanced quality of life. Subscale analysis showed a considerable improvement in the general subscale, a smaller improvement in the social subscale, and no change in the physical score. GBI scores were comparable between 12 weeks and 12 months post-surgery.

Similarly, the GHSI significantly improved HRQoL for total, general, and physical scores when comparing unaided presurgery results with both 6- and 36-month aided post-surgery outcomes (p ≤ 0.007). Social scores showed little or no improvement.

Results from the APHAB questionnaire indicated a significant improvement in global scores and ease of communication (EC), background noise (BN), and reverberation (RV) subscales when comparing unaided presurgery to both 6- and 36-month aided post-surgery. Also, when comparing GHSI and APHAB results with an aided baseline, significant improvements were seen in total and general scores (GHSI) and on global, EC, BN, and RV scores (APHAB).

No apparent significant differences were observed in HRQoL between groups.

DISCUSSION

Key Findings

This study is the first to evaluate the long-term clinical and quality of life (QoL) outcomes of modified minimally invasive Ponto surgery (m-MIPS) and compare these to the original minimal invasive Ponto surgery (o-MIPS) and linear incision technique with tissue preservation (LIT-TP) outcomes. Regarding intra- and postoperative complications, m-MIPS and LIT-TP are safe surgical techniques for inserting BAHIs. Implant survival after 36-month follow-up was comparable between groups, with an implant survival rate of 96% for m-MIPS, 100% for LIT-TP, and a statistically nonsignificant lower implant survival rate of 88% for o-MIPS. The o-MIPS drill was replaced by the modified (m-MIPS) drill in 2018.

Adverse skin reactions across visits were more prevalent after the LIT-TP than after MIPS. Sensibility problems and numbness recovered (almost) entirely after both MIPS and LIT-TP, albeit more rapidly after MIPS. At 36 months post-surgery, device use was comparable between groups, with a mean of 82% of daily users. Regardless of surgical technique, HRQoL significantly improved post-surgery based on the GBI-, GHSI-, and APHAB questionnaires.

Interpretation of Findings

In a preclinical comparison of m-MIPS and o-MIPS drill design, the modified drills generated significantly less heat, except in cases of impaired irrigation (11). This aligns with previously published 6-month data showing (a low) 4% implant loss rate for m-MIPS compared with 12% for o-MIPS (12). Earlier reports on o-MIPS indicated that implant losses occur within 3 months post-implantation (6–9). The current study underlines this by showing no implant loss in any group from 3 months up to 3 years post-surgery. This supports the hypothesis that early implant loss after o-MIPS may result from impaired osseointegration due to overheated bone. The combination of modified drill design and utilization of the three-step drilling protocol with adequate irrigation appears to be vital in enhancing implant survival.

Regarding ISQ measurements, the mean 0- to 6-month AUC of the ISQ-high was significantly lower for MIPS than LIT-TP, likely relating to drilling hole characteristics and not reflecting implant stability per se. The interpretation of individual ISQ-values is debated (21). However, the trend in ISQ values over time is considered a more relevant measure of implant stability where distinct intra-individual decreases may predict loosening of osseointegration. In all three study groups, both ISQ-low and -high did increase over time, leveling out after 12 months.

Compared with LIT-TP, current outcomes on skin sensibility and soft tissue status after MIPS align with the outcomes of previous studies comparing the two surgical techniques (6,9,12,22–24). Sensibility in the skin surrounding the abutment did not significantly decrease in MIPS patients and returned to baseline after 12 weeks. In contrast, baseline levels were reached after 6 months for LIT-TP patients. Subjective numbness was significantly superior for both MIPS groups until 12 months, whereas it was comparable between MIPS and the LIT-TP at the end of the study at 36 months. These results indicate that numbness is no longer a permanent postoperative side effect after BAHI surgery when using tissue-preserving techniques like MIPS. The faster return to normal sensitivity in MIPS is likely due to the minimally invasive approach, with limited trauma to soft tissue during surgery.

In agreement with previous studies (25), this study affirms a trend toward fewer skin reactions after MIPS compared with the LIT-TP. Significantly lower maximum Holgers and IPS scores and fewer adverse scores for both o- and m-MIPS confirm this trend, reflecting less tissue damage and more intact vascularity surrounding the implant.

Another recurring trend observed in this study is reporting more adverse IPS scores than adverse Holgers scores. The I score, which includes “skin integrity,” was significantly higher for MIPS 7and 21 days after surgery. As previously reported (9,12), a common feature is a small gap between the skin and the abutment (wound dehiscence) after MIPS, making the “skin integrity” variable in the IPS stand out in initial surgical follow-ups. The dehiscences, possibly caused by a shift of the skin biopsy during cannula insertion in MIPS, healed within 12 weeks without correlation to other clinical complications and needed no medical treatment (after standard terra cortril treatment postoperatively). At 6 months, the I score was significantly higher for the LIT-TP, corresponding with higher Holgers scores at that time point, indicating erythema, edema, or granulation tissue. At 12 weeks, as previously reported (9,12), but also at 24 months, the S score was significantly higher for the LIT-TP (all S1), indicating increased skin height without needing revision surgery.

In the decision-making process between m-MIPS and the LIT-TP for BAHI implantation, considerations such as costs, training, surgical experience, and individual surgeon's (and perhaps patients') preferences are crucial. MIPS offers 70% shorter surgical time than the LIT-TP, potentially reducing costs. The fast and noninvasive procedure can easily be performed under local anesthesia, which makes MIPS suitable for BAHI implantations outside the main operating room (OR). New developments like MONO (26), a one-step drilling minimally invasive procedure with a novel drill design that ensures even more efficient bone removal and cooling through the cannula, further simplify BAHI implantations and aid transfer of BAHI implantations to a location outside of the operating theater. However, MIPS/MONO may be less suitable for certain cases, such as pediatric patients or those with thin cranial bones or cranial malformations. Experience with LIT-TP is essential during MIPS, as conversion to an open technique could be required, particularly in cases of major bleeding (27).

Strengths and Limitations

The present study is the first to investigate long-term clinical outcomes and QoL after MIPS compared with the LIT-TP and the first to compare m-MIPS and o-MIPS. Strengths include strict protocol adherence and minimal missing data over the 36-month duration. Despite a 5-year gap between operations in the m-MIPS and the LIT-TP groups, all data (excluding IPS scores in the LIT-TP group) were prospectively assessed in a controlled setting, ensuring robust comparability with identical follow-up schedules and outcome measures. However, using a control group from a previous study does bring certain limitations, e.g., precluding randomization, blinded follow-up, and a solid power calculation for sample size. Efforts to navigate challenges posed by the COVID-19 pandemic during the last 1 to 2 years of the MIPS studies were successful, maintaining visit windows and incorporating home-based visits. Although unlikely to have impacted overall outcomes, the pandemic may have influenced long-term (36 months) GHSI and APHAB results due to reduced social interactions and sound environment diversity.

CONCLUSION

Compared with the LIT-TP, m-MIPS showed comparable long-term implant survival, fewer (adverse) skin reactions, and earlier sensibility and numbness recovery. M-MIPS results in favorable clinical and QoL outcomes with low intra- and postoperative complication rates and is considered a safe technique for inserting BAHIs. Moreover, with a shorter surgery time and the ability to operate under local anesthesia in an outpatient setting, m-MIPS appears to be an excellent and lower-cost alternative to LIT-TP.